Search

The synthesis and characterization of the double perovskite SrLaLiOsO6 is presented. It is isostructural (P21/n) and isoelectronic (5d2) with SrLaMgReO6, which has been reported previously. The cell volumes are the same to within 1.4%: i.e., these perovskites are doppelgangers. In a previous study SrLaMgReO6 showed no sign of spin order to 2 K. New data at lower temperatures disclose a maximum in the dc susceptibility near 1.5 K. As the Curie-Weiss (C-W) temperature (Θ) for this material is -161 K, an enormous frustration index, f ≈ 100, is implied (f = |Θ|/Tord). On the other hand, SrLaLiOsO6 does not follow the C-W law over the investigated susceptibility range, 2-300 K. Fitting with an added temperature independent term (TIP) gives μeff = 1.96 μB, Θ = -102 K, and TIP = 1.01 × 10-3 emu/mol. A clear zero-field-cooled (ZFC), field-cooled (FC) divergence in the dc data occurs at ∼10 K, suggesting a much reduced frustration index, f ≈ 10, relative to SrLaMgReO6. The real part of the ac susceptibility data, χ'max, shows a frequency shift that is consistent with a spin glass ground state according to the Mydosh criterion. Heat capacity data for SrLaLiOsO6 show no sign of a λ peak at 10 K and a linear dependence on temperature below 10 K, also supporting a spin glass ground state. A spin frozen ground state for SrLaMgReO6 could not be established from χ' data due to a much weaker signal. Nonetheless, the 10-fold difference in f between these doppelganger materials is remarkable. It is possible that the enhanced covalency with the oxide ligands for Os6+ relative to Re5+ plays a major role here. Finally, a comparison with isostructural La2LiReO6 (with a much smaller f ≈ 4) is made and a correlation between the frustration level and the sense of the local distortion of the Re(Os)-O octahedron is pointed out.

The synthesis and characterization of the double perovskite SrLaLiOsO

${\mathrm{Ba}}_{2}\mathrm{Y}\mathrm{Re}{\mathrm{O}}_{6}$ is a double perovskite material where the ${\mathrm{Re}}^{5+}$ (${d}^{2}$) ions occupy a frustrated face-centered cubic lattice. Despite strong antiferromagnetic interactions between the Re ions, as indicated by a large negative Weiss constant $\ensuremath{\theta}=\ensuremath{-}616$ K, spin freezing in ${\mathrm{Ba}}_{2}\mathrm{Y}\mathrm{Re}{\mathrm{O}}_{6}$ only occurs at 45 K. Since no long-range order of magnetic dipoles has previously been found in either muon spin rotation or neutron diffraction experiments below this temperature, it has been assumed that the low-temperature state is a spin glass. In stark contrast with these findings, we here show that ${\mathrm{Ba}}_{2}\mathrm{Y}\mathrm{Re}{\mathrm{O}}_{6}$ does in fact order with a strongly reduced dipole moment ${\ensuremath{\mu}}_{0}=0.29--0.42\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ via a polarized neutron diffraction experiment on a powder sample. Using the symmetries of the two likeliest magnetic structures and the properties of the $5{d}^{2}$ configuration in the presence of strong spin-orbit coupling and strong crystal fields, we use a recently derived single-ion wave function for ${\mathrm{Re}}^{5+}$ to predict a large quadrupolar moment based on the experimental ${\ensuremath{\mu}}_{o}$.

$\mathrm{L}{\mathrm{a}}_{0.4}\mathrm{C}{\mathrm{e}}_{0.6}\mathrm{C}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$ represents a borderline case in the range of solid solutions formed in the pseudobinary system $\mathrm{LaC}{\mathrm{o}}_{2}{\mathrm{P}}_{2}\text{\ensuremath{-}}\mathrm{CeC}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$. The material undergoes ferromagnetic ordering at \ensuremath{\sim}225 K followed by a structural collapse at \ensuremath{\sim}190 K, which leads to a strong suppression of magnetization. The structural phase transition manifests itself in a gradual decrease in the parameter $c$ and a relatively smaller increase of the parameter $a$ of the tetragonal lattice. Interestingly, a combination of magnetic measurements and nonpolarized and polarized neutron scattering experiments suggests that the structural collapse does not lead to an antiferromagnetically ordered state, observed in samples with the higher Ce content. On the contrary, $\mathrm{L}{\mathrm{a}}_{0.4}\mathrm{C}{\mathrm{e}}_{0.6}\mathrm{C}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$ appears to enter a disordered, spin glass state, with gradual dissipation of the ferromagnetic ordering taking place simultaneously with the structural collapse, as evidenced by temperature-dependent measurements of the depolarization factor for a polarized neutron beam passing through the sample. The observed behavior is analogous to that reported for so-called reentrant spin glasses. In the present case, however, the appearance of the reentrant spin glass regime is caused not by tuning the chemical composition but by the structural phase transition. Electronic structure calculations confirm that the loss of magnetic ordering is caused by the subtle change to the density of states at the Fermi level due to the variation of the crystal structure of the material.

Double perovskites (DP) of the general formula Ba2MReO6, where M = Mg, Zn, and Y2/3, all based on Re6+ (5d1, t2g1), were synthesized and studied using magnetization, heat capacity, muon spin relaxation, and neutron-scattering techniques. All are cubic, Fm3m, at ambient temperature to within the resolution of the X-ray and neutron diffraction data, although the muon data suggest the possibility of a local distortion for M = Mg. The M = Mg DP is a ferromagnet, Tc = 18 K, with a saturation moment ∼0.3 bohr magnetons at 3 K. There are two anomalies in the heat capacity: a sharp feature at 18 K and a broad maximum centered near 33 K. The total entropy loss below 45 K is 9.68 e.u., which approaches R ln 4 (11.52 e.u.) supporting a j = 3/2 ground state. The unit cell constants of Ba2MgReO6 and the isostructural, isoelectronic analogue, Ba2LiOsO6, differ by only 0.1%, yet the latter is an anti-ferromagnet. The M = Zn DP also appears to be a ferromagnet, Tc = 11 K, μsat(Re) = 0.1 μB. In this case the heat capacit...

We demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in ACo2As2 (A = Eu and Ca) has a general consequence of causing these antiferromagnetic materials to become ferromagnets. In all cases, the mixed valence triggered at the electropositive A site results in the increase of the Co 3d density of states at the Fermi level. Remarkably, the dramatic alteration of magnetic behavior results from the very minor (0.15 electron) change in the population of the 3d orbitals. The mixed valence state of Eu observed in the high-pressure (HP) form of EuCo2As2 exhibits a remarkable stability, achieving the average oxidation state of +2.25 at 12.6 GPa. In the case of CaCo2As2, substituting even 10% of Eu or La into the Ca site causes ferromagnetic ordering of Co moments. Similar to HP-EuCo2As2, the itinerant 3d ferromagnetism emerges from electronic doping into the Co layer because of chemical compression of Eu sites in Ca0.9Eu0.1Co1.91As2 or direct electron doping in Ca0.85La0.15Co1.89As2. The results reported herein demonstrate the general possibility of amplifying minor localized electronic effects to achieve major changes in material's properties via involvement of strongly correlated electrons.

A magnetic solid with weak spin frustration tends to adopt a noncollinear magnetic structure such as cycloidal structure below a certain temperature and a spin density wave (SDW) slightly above this temperature. The causes for these observa-tions were explored by studying the magnetic structures of BaYFeO4, which undergoes an SDW and a cycloidal phase transi-tion below 48 and 36 K, respectively, in terms of density func-tional theory calculations. We show that an SDW structure aris-es from a superposition of two magnetic states of opposite chi-rality, an SDW state precedes a chiral magnetic state due to the lattice relaxation, and whether an SDW is transversal or longitudinal is governed by the magnetic anisotropy of magnetic ions., Comment: 4 pages, 4 figures not including the SI

A series of colloidal MxFe3–xO4 (M = Mn, Co, Ni; x = 0–1) nanoparticles with diameters ranging from 6.8 to 11.6 nm was synthesized by hydrothermal reaction in aqueous medium at low temperature (200 °C). Energy-dispersive X-ray microanalysis and inductively coupled plasma spectrometry confirm that the actual elemental compositions agree well with the nominal ones. The structural properties of the obtained nanoparticles were investigated by powder X-ray diffraction, Raman spectroscopy, Mossbauer spectroscopy, X-ray and neutron pair distribution function analysis, and electron microscopy. The results demonstrate that our synthesis technique leads to the formation of chemically uniform single-phase solid solution nanoparticles with cubic spinel structure, confirming intrinsic doping. The local structure of the Fe3O4 NPs is distorted with respect to the cubic inverse-spinel structure, while chemical substitution of Fe by Mn or Ni partially eliminates the local distortions. Magnetic studies showed that, in comp...

Neutron diffraction and muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) studies are presented for the newly characterized polymorph of ${\mathrm{NiNb}}_{2}{\mathrm{O}}_{6} (\ensuremath{\beta}\ensuremath{-}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{6})$ with space group ${P4}_{2}/n$ and $\ensuremath{\mu}\mathrm{SR}$ data only for the previously known columbite structure polymorph with space group $Pbcn$. The magnetic structure of the ${P4}_{2}/n$ form was determined from neutron diffraction using both powder and single-crystal data. Powder neutron diffraction determined an ordering wave vector $\stackrel{P\vec}{k}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$. Single-crystal data confirmed the same $\stackrel{P\vec}{k}$ vector and showed that the correct magnetic structure consists of antiferromagnetically coupled chains running along the $a$ or $b$ axis in adjacent ${\mathrm{Ni}}^{2+}$ layers perpendicular to the $c$ axis, which is consistent with the expected exchange interaction hierarchy in this system. The refined magnetic structure is compared with the known magnetic structures of the closely related trirutile phases, ${\mathrm{NiSb}}_{2}{\mathrm{O}}_{6}$ and ${\mathrm{NiTa}}_{2}{\mathrm{O}}_{6}$. $\ensuremath{\mu}\mathrm{SR}$ data finds a transition temperature of ${T}_{N}\ensuremath{\sim}15\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for this system, while the columbite polymorph exhibits a lower ${T}_{N}=5.7(3)$ K. Our $\ensuremath{\mu}\mathrm{SR}$ measurements also allowed us to estimate the critical exponent of the order parameter $\ensuremath{\beta}$ for each polymorph. We found $\ensuremath{\beta} =0.25(3)$ and 0.16(2) for the $\ensuremath{\beta}$ and columbite polymorphs, respectively. The single-crystal neutron scattering data give a value for the critical exponent $\ensuremath{\beta} =0.28(3)$ for $\ensuremath{\beta}\ensuremath{-}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{6}$, in agreement with the $\ensuremath{\mu}\mathrm{SR}$ value. While both systems have $\ensuremath{\beta}$ values less than 0.3, which is indicative of reduced dimensionality, this effect appears to be much stronger for the columbite system. In other words, although both systems appear to be well described by $S=1$ spin chains, the interchain interactions in the $\ensuremath{\beta}$ polymorph are likely much larger.

We report resonant inelastic X-ray scattering (RIXS) measurements on ordered double perovskite samples containing Re$^{5+}$ and Ir$^{5+}$ with $5d^2$ and $5d^4$ electronic configurations respectively. In particular, the observed RIXS spectra of Ba$_2$YReO$_6$ and Sr$_2$MIrO$_6$ (M=Y, Gd) show sharp intra-t$_{2g}$ transitions, which can be quantitatively understood using a minimal `atomic' Hamiltonian incorporating spin-orbit coupling ($\lambda$) and Hund's coupling ($J_H$). Our analysis yields $\lambda=0.38(2)$eV with $J_H=0.26(2)$eV for Re$^{5+}$, and $\lambda=0.42(2)$eV with $J_H=0.25(4)$eV for Ir$^{5+}$. Our results provide the first sharp estimates for the Hund's coupling in $5d$ oxides, and suggest that it should be treated on equal footing with spin-orbit interaction in multi-orbital $5d$ transition metal compounds., Comment: Added theoretical RIXS spectrum + expanded. To appear in PRB

$\mathrm{L}{\mathrm{a}}_{0.25}\mathrm{P}{\mathrm{r}}_{0.75}\mathrm{C}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$ crystallizes in the tetragonal $\mathrm{ThC}{\mathrm{r}}_{2}\mathrm{S}{\mathrm{i}}_{2}$ structure type and shows multiple magnetic phase transitions driven by changes in temperature and magnetic field. The nature of these transitions was investigated by a combination of magnetic and magnetoresistance measurements and both single crystal and powder neutron diffraction. The Co magnetic moments order ferromagnetically (FM) parallel to the $c$ axis at 282 K, followed by antiferromagnetic (AFM) ordering at 225 K. In the AFM structure, the Co magnetic moments align along the $c$ axis with FM $[\mathrm{C}{\mathrm{o}}_{2}{\mathrm{P}}_{2}]$ layers arranged in an alternating sequence, $\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$, which leads to the doubling of the $c$ axis in the magnetic unit cell. Another AFM transition is observed at 27 K, due to the ordering of a half of Pr moments in the $ab$ plane. The other half of Pr moments undergoes AFM ordering along the $c$ axis at 11 K, causing simultaneous reorientation of the previously ordered Pr moments into an AFM structure with the moments being canted with respect to the $c$ axis. This AFM transition causes an abrupt decrease in electrical resistivity at 11 K. Under applied magnetic field, two metamagnetic transitions are observed in the Pr sublattice at 0.8 and 5.4 T. They correlate with two anomalies in magnetoresistance measurements at the same critical fields. A comparison of the temperature- and field-dependent magnetic properties of $\mathrm{L}{\mathrm{a}}_{0.25}\mathrm{P}{\mathrm{r}}_{0.75}\mathrm{C}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$ to the magnetic behavior of $\mathrm{PrC}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$ is provided.

Neutron pair distribution function data are used to investigate the local structures of two oxygen deficient perovskites with simple cubic crystal structures. La

Four rare-earth cobalt arsenides, $RCo_{2}As_{2}$ (R = La, Ce, Pr, Nd), were obtained by reactions of constituent elements in molten Bi. The use of Bi flux also allowed the growth of representative single crystals. All compounds are isostructural and belong to the $ThCr_{2}Si_{2}$ type (space group I4/mmm). The formation of Co vacancies is observed in all structures, while the structures of La- and Ce-containing compounds also show incorporation of minor Bi defects next to the R crystallographic site. Correspondingly, the general formula of these materials can be written as $R_{1–x}Bi_{x}Co_{2−\delta}As_{2}$, with $x/\delta$ = 0.03/0.1, 0.05/0.15, 0/0.2, and 0/0.3 for R = La, Ce, Pr, and Nd, respectively. All compounds exhibit high-temperature ferromagnetic ordering of Co magnetic moments in the range 60–200 K. Electronic band structure calculations revealed a high peak in the density of states at the Fermi level, thus supporting the itinerant nature of magnetism in the Co sublattice. The magnetic ordering in the lanthanide sublattice takes place at lower temperatures, with the R moments aligning antiparallel to the Co moments to give a ferrimagnetic ground state. The measurements on oriented single crystals demonstrated significant magnetic anisotropy in the ferrimagnetic state, with the preferred moment alignment along the $\mathit{c}$ axis of the tetragonal lattice. Neutron powder diffraction failed to reveal the structure of magnetically ordered states but confirmed the presence of Co vacancies. X-ray absorption near-edge structure spectroscopy on $Ce_{1.95}Bi_{0.05}Co_{1.85}As_{2}$ showed the average oxidation state of Ce to be +3.06. Solid state NMR spectroscopy revealed a substantially reduced hyperfine field on the Co atoms in the vicinity of Bi defects.

The novel iron-based compound, BaYFeO4, crystallizes in the Pnma space group with two distinct Fe(3+) sites, that are alternately corner-shared [FeO5](7-) square pyramids and [FeO6](9-) octahedra, forming into [Fe4O18](24-) rings, which propagate as columns along the b-axis. A recent report shows two discernible antiferromagnetic (AFM) transitions at 36 and 48 K in the susceptibility, yet heat capacity measurements reveal no magnetic phase transitions at these temperatures. An upturn in the magnetic susceptibility measurements up to 400 K suggests the presence of short-range magnetic behavior at higher temperatures. In this Article, variable-temperature neutron powder diffraction and high-temperature magnetic susceptibility measurements were performed to clarify the magnetic behavior. Neutron powder diffraction confirmed that the two magnetic transitions observed at 36 and 48 K are due to long-range magnetic order. Below 48 K, the magnetic structure was determined as a spin-density wave (SDW) with a propagation vector, k = (0, 0, (1)/3), and the moments along the b-axis, whereas the structure becomes an incommensurate cycloid [k = (0, 0, ∼0.35)] below 36 K with the moments within the bc-plane. However, for both cases the ordered moments on Fe(3+) are only of the order ∼3.0 μB, smaller than the expected values near 4.5 μB, indicating that significant components of the Fe moments remain paramagnetic to the lowest temperature studied, 6 K. Moreover, new high-temperature magnetic susceptibility measurements revealed a peak maximum at ∼550 K indicative of short-range spin correlations. It is postulated that most of the magnetic entropy is thus removed at high temperatures which could explain the absence of heat capacity anomalies at the long-range ordering temperatures. Published spin dimer calculations, which appear to suggest a k = (0, 0, 0) magnetic structure, and allow for neither low dimensionality nor geometric frustration, are inadequate to explain the observed complex magnetic structure.

AlFe2B2 was prepared by two alternative synthetic routes, arc melting and synthesis from Ga flux. In the layered crystal structure, infinite chains of B atoms are connected by Fe atoms into two-dimensional [Fe2B2] slabs that alternate with layers of Al atoms. As expected from the theoretical analysis of electronic band structure, the compound exhibits itinerant ferromagnetism, with the ordering temperature of 307 K. The measurement of magnetocaloric effect (MCE) as a function of applied magnetic field reveals isothermal entropy changes of 4.1 J kg(-1) K(-1) at 2 T and 7.7 J kg(-1) K(-1) at 5 T. These are the largest values observed near room temperature for any metal boride and for any magnetic material of the vast 122 family of layered structures. Importantly, AlFe2B2 represents a rare case of a lightweight material prepared from earth-abundant, benign reactants which exhibits a substantial MCE while not containing any rare-earth elements.

Neutron pair distribution function data are used to investigate the local structures of two oxygen deficient perovskites with simple cubic crystal structures. La0.5Ba0.5MnO2.5 is found to have alternating layers of MnO6 octahedra and MnO4 tetrahedra, making it similar to other members of the La1-xBaxMnO2.5 series with smaller x values which have brownmillerite crystal structures. Our fitting results suggest that La0.5Ba0.5MnO2.5 is not locally similar to any one brownmillerite structure type but instead has a variety of intra-layer and inter-layer relationships. We propose that this could be due to short range segregation of the much differently sized La(3+) and Ba(2+) cations, which creates significantly different inter-layer distances. BaY0.25Fe0.75O2.5 is found not to have a brownmillerite-type local structure but rather seems to consist of structural units which are similar to Ba3YFe2O7.5, an earlier member of the BanYFen-1O2.5n series. The PDF analysis shows that there are never neighboring Y atoms and that the O vacancies lie exclusively between Fe atoms, such that Y is always octahedrally coordinated. The PDF also suggests that there are more tetrahedral Fe than expected, which could be due to the presence of terminal O atoms in Fe centered dimeric units, similar to what is found in Ba3YFe2O7.5.

Three closely related double perovskites (DP) based on the $5{d}^{2}$ ion, $\mathrm{O}{\mathrm{s}}^{6+},\mathrm{B}{\mathrm{a}}_{2}M\mathrm{Os}{\mathrm{O}}_{6}$, with $M=\mathrm{Mg}$, Zn, and Cd have been prepared and characterized using x-ray (XRD) and neutron diffraction (ND), dc magnetization, heat capacity, and muon spin relaxation $(\ensuremath{\mu}\mathrm{SR})$ techniques. All three are cubic, Fm-$3m$, at ambient temperature from XRD with $\mathrm{\ensuremath{\Delta}}d/d\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ resolution. For both $M=\mathrm{Mg}$ and Zn, ND data at 3.5 K and lower, $\mathrm{\ensuremath{\Delta}}d/d=2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, resolution show no signs of a distortion. The results are compared with the known DP material, $\mathrm{B}{\mathrm{a}}_{2}\mathrm{CaOs}{\mathrm{O}}_{6}$, which shows antiferromagnetic (AF) order below ${T}_{\mathrm{N}}=49\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and a moderate frustration index, $f\ensuremath{\sim}4$, $(f=|{\ensuremath{\theta}}_{\mathrm{CW}}|/{T}_{\mathrm{N}})$, where ${\ensuremath{\theta}}_{\mathrm{CW}}$ is the Curie-Weiss temperature. $\mathrm{B}{\mathrm{a}}_{2}\mathrm{MgOs}{\mathrm{O}}_{6}$ with a unit cell constant ${a}_{0}=8.0757(1)\phantom{\rule{0.16em}{0ex}}\AA{}$, 3% smaller than for $\mathrm{B}{\mathrm{a}}_{2}\mathrm{CaOs}{\mathrm{O}}_{6}$, also shows N\'eel order below ${T}_{\mathrm{N}}=51\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with $f\ensuremath{\sim}2$. However, $\mathrm{B}{\mathrm{a}}_{2}\mathrm{ZnOs}{\mathrm{O}}_{6},{a}_{0}=8.0975(1)\phantom{\rule{0.16em}{0ex}}\AA{}$, 0.27% larger than $\mathrm{B}{\mathrm{a}}_{2}\mathrm{MgOs}{\mathrm{O}}_{6}$, does not show N\'eel order from either heat capacity or $\ensuremath{\mu}\mathrm{SR}$ data. A zero field cooled/field cooled (ZFC/FC) irreversibility occurs near 30 K and a broad heat capacity anomaly is detected at a similar temperature. The $\ensuremath{\mu}\mathrm{SR}$ data are consistent with a weak spin ordering with an onset below 28 K but with a coexisting dynamic component. $\mathrm{B}{\mathrm{a}}_{2}\mathrm{CdOs}{\mathrm{O}}_{6}$ with ${a}_{0}=8.3190(1)\phantom{\rule{0.16em}{0ex}}\AA{}$, 0.5% smaller than $\mathrm{B}{\mathrm{a}}_{2}\mathrm{CaOs}{\mathrm{O}}_{6}$, shows no evidence for any type of order/spin freezing to 0.47 K from any of the measurement techniques applied. The results for $M=\mathrm{Zn}$ and Cd appear to lie outside of the mean field theory of Chen and Balents [Phys. Rev. B 84, 094420 (2011)] for cubic ${d}^{2}$ DP with strong spin orbit coupling, but $\mathrm{B}{\mathrm{a}}_{2}\mathrm{MgOs}{\mathrm{O}}_{6}$, along with $\mathrm{B}{\mathrm{a}}_{2}\mathrm{CaOs}{\mathrm{O}}_{6}$, is likely one of the three predicted AF phases. The remarkable contrast between the doppelg\"anger pairs $M=\mathrm{Mg}$/Zn and $M=\mathrm{Ca}$/Cd may be traceable to differences in electronic structure of the diamagnetic $M$ ions. All of the super-super exchange pathways in these DP materials involve Os--O--$M$--O--Os linkages.

Double perovskites (DP) of the general formula Ba

The frustrated double perovskite $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$, based on $\mathrm{O}{\mathrm{s}}^{5+}\phantom{\rule{0.16em}{0ex}}(5{d}^{3},\phantom{\rule{0.16em}{0ex}}{\mathrm{t}}_{2}^{3})$ is studied using magnetization, elastic neutron scattering, heat capacity, and muon spin relaxation (\ensuremath{\mu}SR) techniques and compared with isostructural $(P{2}_{1}/n)\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6},\phantom{\rule{0.16em}{0ex}}\mathrm{R}{\mathrm{u}}^{5+}(4{d}^{3},\phantom{\rule{0.16em}{0ex}}{\mathrm{t}}_{2}^{3})$. While previous studies of $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$ showed a broad susceptibility maximum $({\ensuremath{\chi}}_{\mathrm{max}})$ near 40 K, heat capacity data indicate a sharp peak at 30 K, similar to $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6}$ with ${\ensuremath{\chi}}_{\mathrm{max}}\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and a heat capacity peak at 24 K. Significant differences between the two materials are seen in powder neutron diffraction where the magnetic structure is described by $\mathbit{k}=(1/2\phantom{\rule{0.16em}{0ex}}1/2\phantom{\rule{4pt}{0ex}}0)$ for $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$, while $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6}$ has been reported with $\mathbit{k}=(000)$, structure for face centered lattices. For the $\mathbit{k}=(1/2\phantom{\rule{0.16em}{0ex}}1/2\phantom{\rule{4pt}{0ex}}0)$ structure, one has antiferromagnetic layers stacked antiferromagnetically, while for $\mathbit{k}=(0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}0)$ structure, ferromagnetic layers are stacked antiferromagnetically. In spite of these differences, both can be considered as type I fcc antiferromagnetic structures. For $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$, the magnetic structure is best described in terms of linear combinations of basis vectors belonging to irreducible representations ${\mathrm{\ensuremath{\Gamma}}}_{2}$ and ${\mathrm{\ensuremath{\Gamma}}}_{4}$. The combinations ${\mathrm{\ensuremath{\Gamma}}}_{2}--{\mathrm{\ensuremath{\Gamma}}}_{4}$ and ${\mathrm{\ensuremath{\Gamma}}}_{2}+{\mathrm{\ensuremath{\Gamma}}}_{4}$ could not be distinguished from refinement of the data. In all cases, the $\mathrm{O}{\mathrm{s}}^{5+}$ moments lie in the $yz$ plane with the largest component along $y$. The total moment is 1.81(4) ${\ensuremath{\mu}}_{\mathrm{B}}$. For $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6}$, the $\mathrm{R}{\mathrm{u}}^{5+}$ moments are reported to lie in the $xz$ plane. In addition, while neutron diffraction, \ensuremath{\mu}SR and NMR data indicate a unique ${T}_{\mathrm{N}}=24\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiRu}{\mathrm{O}}_{6}$, the situation for $\mathrm{L}{\mathrm{a}}_{2}\mathrm{LiOs}{\mathrm{O}}_{6}$ is more complex, with heat capacity, neutron diffraction, and \ensuremath{\mu}SR indicating two ordering events at 30 and 37 K, similar to the cases of cubic $\mathrm{B}{\mathrm{a}}_{2}\mathrm{YRu}{\mathrm{O}}_{6}$ and monoclinic $\mathrm{S}{\mathrm{r}}_{2}\mathrm{YRu}{\mathrm{O}}_{6}$.

The features of near-edge structure around the L 3-Eu absorption edge are investigated in EuCo2P2, Pr1 − x Eu x Co2P2 (x = 0.2, 0.4), and Nd0.6Eu0.4Co2P2 by means of resonant inelastic X-ray scattering (RIXS). Europium is shown to be in the intermediate valence state in all compounds. A comparison is made of the values of europium intermediate valence obtained by different methods employing the synchrotron radiation. The correlation between europium valence and the observed changes of magnetic ordering in the systems under investigation is discussed.

Magnetic properties of GdCo3P2 and GdCo5P3 are reported. Both compounds are paramagnetic at room temperature. GdCo3P2 undergoes an antiferromagnetic transition due to the ordering of gadolinium magnetic moments at 16 K. The cobalt atoms do not contribute to the magnetism of this compound. The antiferromagnetic transition is shifted to lower temperatures by an applied magnetic field, reaching 12 K at 0.60 T. Such behavior is attributed to spin canting or spin reorientation at higher fields. No such abrupt transition is observed in GdCo5P3, but the field dependence of magnetization at 1.8 K indicates canted antiferromagnetism. In addition, observation of distinct peaks in the AC magnetic susceptibility at 66 K, 54 K, and 48 K suggests occurrence of successive magnetic state changes. An examination of previously determined crystal structures of these solids reveals common structural elements derived from the [Co2P2] layer present in the structure of RECo2P2 phases (RE = rare earth; ThCr2Si2 structure type). In GdCo3P2 and GdCo5P3, such layers are truncated and form triangular patterns when viewed along the [010] crystallographic direction. Single-crystal X-ray diffraction analysis showed that GdCo5P3 is isostructural to YCo5P3. The gadolinium atoms form chains along [010] with d(Gd–Gd) = 3.683 A. In GdCo3P2, the gadolinium atoms form cobalt-centered trigonal prisms with d(Gd–Gd) approx. 3.7–3.8 A. The prisms share their trigonal faces to form columns along [010].

To explore the evolution of magnetic properties from ferromagnetic LaCo(2)P(2) to paramagnetic LaFe(2)P(2) (both of ThCr(2)Si(2) structure type) a series of mixed composition LaFe(x)Co(2-x)P(2) (x ≤ 0.5) has been comprehensively investigated by means of single-crystal and powder X-ray and neutron diffraction, magnetization and heat capacity measurements, Mössbauer spectroscopy, and electronic band structure calculations. The Curie temperature decreases from 132 K in LaCo(2)P(2) to 91 K in LaFe(0.05)Co(1.95)P(2). The ferromagnetic ordering is suppressed at higher Fe content. LaFe(0.1)Co(1.9)P(2) and LaFe(0.2)Co(1.8)P(2) demonstrate spin-glass-like behavior, which was also confirmed by the absence of characteristic features of long-range magnetic ordering, namely, a λ-type anomaly in the heat capacity, a hyperfine splitting in the Mössbauer spectrum, and magnetic reflections in the neutron diffraction pattern. Finally, both LaFe(0.3)Co(1.7)P(2) and LaFe(0.5)Co(1.5)P(2) exhibit paramagnetic behavior down to 1.8 K. The unit cell parameters of the mixed compounds do not follow the Vegard behavior as the increase in the Fe content results in the decrease of average M-M distances (M = Fe, Co). Quantum-chemical calculations and crystal orbital Hamiltonian population analysis reveal that upon aliovalent (nonisoelectronic) substitution of Fe for Co the antibonding character of M-M interactions is reduced while the Fermi level is shifted below the DOS peak in the 3d metal subband. As the result, at higher Fe content the Stoner criterion is not satisfied and no magnetic ordering is observed.

A family of rare-earth cobalt phosphides La1−xPrxCo2P2 (0 ≤ x ≤ 1) that belong to the ThCr2Si2 structure type has been prepared and characterized by structural and magnetic methods and electronic band structure calculations. All studied quaternary phases exhibit multiple magnetic transitions, leading to an observation of ferro- and metamagnetism and magnetic pole reversal. The ferromagnetic transition temperature of LaCo2P2 (132 K) increases dramatically upon substitution of Pr for La and reaches 268 K for La0.25Pr0.75Co2P2. This increase is accompanied by elongation of intralayer Co−Co distances. Variable temperature X-ray diffraction data revealed that temperature dependences of unit cell parameters, and, correspondingly, both intra- and interlayer Co−Co separations show anomalous changes at temperatures close to the ferromagnetic transition. The electronic structure calculations reveal a strong peak in the nonmagnetic density of states (DOS). This instability is removed in the spin-polarized DOS due to...

We report the crystal structure, magnetization, and neutron scattering measurements on the double perovskite ${\mathrm{Ba}}_{2}{\mathrm{YOsO}}_{6}$. The $Fm\overline{3}m$ space group is found both at 290 K and 3.5 K with cell constants ${a}_{0}=8.3541(4)$ \AA{} and $8.3435(4)$ \AA{}, respectively. ${\mathrm{Os}}^{5+} (5{d}^{3})$ ions occupy a nondistorted, geometrically frustrated face-centered-cubic (fcc) lattice. A Curie-Weiss temperature $\ensuremath{\theta}\ensuremath{\sim}\ensuremath{-}700$ K suggests the presence of a large antiferromagnetic interaction and a high degree of magnetic frustration. A magnetic transition to long-range antiferromagnetic order, consistent with a type-I fcc state below ${T}_{\mathrm{N}}\ensuremath{\sim}69$ K, is revealed by magnetization, Fisher heat capacity, and elastic neutron scattering, with an ordered moment of 1.65(6) ${\ensuremath{\mu}}_{B}$ on ${\mathrm{Os}}^{5+}$. The ordered moment is much reduced from either the expected spin-only value of $\ensuremath{\sim}3 {\ensuremath{\mu}}_{B}$ or the value appropriate to $4{d}^{3} {\mathrm{Ru}}^{5+}$ in isostructural ${\mathrm{Ba}}_{2}{\mathrm{YRuO}}_{6}$ of 2.2(1) ${\ensuremath{\mu}}_{B}$, suggesting a role for spin-orbit coupling (SOC). Triple-axis neutron scattering measurements of the order parameter suggest an additional first-order transition at $T=67.45$ K, and the existence of a second-ordered state. Time-of-flight inelastic neutron results reveal a large spin gap $\ensuremath{\Delta}\ensuremath{\sim}17$ meV, unexpected for an orbitally quenched, ${d}^{3}$ electronic configuration. We discuss this in the context of the $\ensuremath{\sim}5$ meV spin gap observed in the related ${\mathrm{Ru}}^{5+},\phantom{\rule{0.16em}{0ex}}4{d}^{3}$ cubic double perovskite ${\mathrm{Ba}}_{2}{\mathrm{YRuO}}_{6}$, and attribute the $\ensuremath{\sim}3$ times larger gap to stronger SOC present in this heavier, $5d$, osmate system.

Four new double perovskites, SrLaMReO(6) (M = Mg, Mn, Co, Ni) in which Re(5+) (5d(2)) is present, were prepared via conventional solid state reactions and characterized by X-ray and neutron powder diffraction, XANES, SQUID magnetometry, and muon spin relaxation (μSR). Synchrotron X-ray and neutron diffraction experiments confirmed that all compounds crystallize in the monoclinic P2(1)/n structure type, which consists of alternately corner-shared octahedra of MO(6) and ReO(6). Rietveld refinement results indicated anti-site mixing of less than 7% on the M/Re sites. Bond valence sum calculations (BVS) suggest all M and Re ions are 2+ and 5+, respectively, and for the Mn-containing phase this is also supported by XANES measurements. All of the materials are paramagnetic at room-temperature and their Curie-Weiss temperatures are positive (except for Mg) indicating net ferromagnetic interactions. No evidence for long-range magnetic order is evident in the dc magnetic susceptibility and μSR measurements for SrLaMgReO(6) to 2 K. The Mn-phase shows long-range order at T(C) = 190 K and neutron diffraction revealed a ferromagnetic structure with a refined net moment of ∼3.7μ(B). Both Co- and Ni-containing phases exhibit spin glass behavior at T(G) = 23 and 30 K, respectively, which is supported by neutron diffraction and a.c. susceptibility data. The structure and physical properties of these four new rhenium based ordered double perovskites are compared to the closely related "pillared perovskites", La(5)Re(3)MO(16), the isoelectronic Os(6+) (5d(2)) double perovskite Sr(2)CoOsO(6), and the Re(6+) (5d(1)) double perovskites, Sr(2)MReO(6), (M = Mg, Ca, Mn, Co, Ni).

Here we investigated diffuse magnetic scattering in the highly frustrated double perovskite Ba2YRuO6 using polarized neutrons. Consistent with previous reports, the material shows two apparent transitions at 47 and 36 K to an eventual type I face-centered-cubic magnetic ground state. The (100) magnetic reflection shows different behavior from the five other observed reflections upon heating from 1.8 K, with the former broadening well beyond the resolution limit near 36 K. Closer examination of the latter group reveals a small, but clear, increase in peak widths between 36 and 47 K, indicating that this regime is dominated by short-range spin correlations. Diffuse magnetic scattering persists above 47 K near the position of (100) to at least 200 K, consistent with strong frustration. Reverse Monte Carlo (RMC) modeling of the diffuse scattering from 45 to 200 K finds that the spin-spin correlations between nearest and next-nearest neighbors are antiferromagnetic and ferromagnetic, respectively, at temperatures near the upper ordering temperature, but both become antiferromagnetic and of similar magnitude above 100 K. The significance of this unusual crossover is discussed in light of the super-superexchange interactions between nearest and next-nearest neighbors in this material and the demands of type I order. The dimensionality ofmore » the correlations is addressed by reconstructing the scattering in the (hk0) plane using the RMC spin configurations. This indicates that one-dimensional spin correlations dominate at temperatures close to the first transition. In addition, a comparison between mean-field calculations and (hk0) scattering implies that further neighbor couplings play a significant role in the selection of the ground state. Finally, the results and interpretation are compared with those recently published for monoclinic Sr2YRuO6, and similarities and differences are emphasized.« less

The oxyfluoride SrFeO 2 F has been prepared via a low temperature route involving the infinite-layer SrFeO 2 and XeF 2 . SrFeO 2 F crystallizes in the cubic space group Pm- 3 m with disordered oxygen and fluorine atoms on the anion site. Recent reports demonstrated that SrFeO 2 F is antiferromagnetic at room temperature and the zero field cooled and field cooled curves diverge at ~150 K and ~60 K, suggesting that the material has a spin glassy magnetic state at low temperatures. In this article, variable-temperature neutron diffraction (4–723 K) was performed to clarify the magnetic behavior observed in this material. Neutron powder diffraction measurements confirmed the antiferromagnetic (AFM) ordering of the system at room temperature. Below 710(1) K, the magnetic structure is a G-type AFM structure characterized by a propagation vector k =( 1 2 , 1 2 , 1 2 ). The ordered moments on Fe 3+ are 4.35(6) µ B at 4 K and 4.04(5) µ B at 290 K. Our results indicate that the cubic structure is retained all the way to base temperature (4 K) in contrast to PbFeO 2 F. These results are compared with those of Pb and Ba analogs which exhibit very similar magnetic behavior. Furthermore, the observation of magnetic reflections at 4 K in the diffraction pattern shows the absence of the previously proposed spin glassy behavior at low temperatures. Previous proposals to explain the ZFC/FC divergences are examined.

The B-site ordered double perovskite Ba2CaOsO6 was studied by d.c. magnetic susceptibility, powder neutron diffraction and muon spin relaxation methods. The lattice parameter is a = 8.3619(6) A at 280K and cubic symmetry (Fm-3m) is retained to 3.5K with a = 8.3426(5) A. Curie-Weiss susceptibility behavior is observed for T > 100K and the derived constants are C = 0.3361(1)emu-K/mole and T_CW = -156.2(3) K, in excellent agreement with literature values. This Curie constant is much smaller than the spin-only value of 1.00 emu-K/mole for a 5d^2 Os6+ configuration, indicating a major influence of spin-orbit coupling. Previous studies had detected both susceptibility and heat capacity anomalies near 50K but no definitive conclusion was drawn concerning the nature of the ground state. While no ordered Os moment could be detected by powder neutron diffraction, muon spin relaxation (muSR) data show clear long-lived oscillations indicative of long-range magnetic order below T_C = 50K. An estimate of the ordered moment on Os6+ is ~0.2 mu_B, based upon a comparison with muSR data for Ba2YRuO6 with a known ordered moment of 2.2 mu_B. These results are compared with those for isostructural Ba2YReO6 which contains Re5+, also 5d^2, and has a nearly identical unit cell constant, a = 8.3628(2) A. In contrast, Ba2YReO6 shows T_CW = -616 K, and a complex spin-disordered and, ultimately, spin-frozen ground state below 50 K, indicating a much higher level of geometric frustration than in Ba2CaOsO6. A comparison is made to recent theory on d^2 double perovskites., 9 pages, 11 figures, 2 tables

BaYFeO4 is prepared by solid state reaction of stoichiometric amounts of BaCO3, Y2O3, and Fe2O3 in air (1.

Clemens et al. reported on the results published by us (Thompson et al. J. Solid State Chem. 219 (2014) 173–178) on the crystal structure of SrFeO{sub 2}F, which they suggest to actually crystallize in the orthorhombic space group Imma rather than the cubic Pm-3m structure at lower temperatures (Clemens et al. J. Solid State Chem. (2015), (http://dx.doi.org/10.1016/j.jssc.2015.02.022)). In this report, we provide evidence to support their claim that at lower temperatures (

Magnetic behavior of La${}_{0.75}$Pr${}_{0.25}$Co${}_{2}$P${}_{2}$ was investigated by a combination of magnetic measurements, magneto-optical imaging, neutron diffraction, and x-ray absorption spectroscopy, including x-ray magnetic circular dichroism. The material crystallizes in the ThCr${}_{2}$Si${}_{2}$ structure type and exhibits three consecutive magnetic phase transitions. At 167 K, the Co magnetic moments order ferromagnetically in the $ab$ plane of the tetragonal crystal structure. At 66 K, a ferromagnetic ordering of Pr(4$f$) moments parallel to the $c$ axis causes a rotation of the Co(3$d$) moments towards the $c$ axis in the direction opposite to the Pr moments, thus forming a noncollinear ferrimagnetically ordered structure and switching the direction of the total magnetization from the $ab$ plane to the $c$ axis. The third magnetic transition observed at 35 K is likely associated with the establishment of the collinear ferrimagnetic order along the $c$ axis.

The title compound is synthesized from the elements either by arc-meltingand subsequent annealing at 1173 K for 7 d or by reaction in a Ga flux (alumina crucible, 1173 K, 7 d).

Reactions between the complexes M3(dpa)4Cl2 (M = Co, Ni; dpa = 2,2′-dipyridylamine), which contain linear trimetallic fragments, and (Bu4N)3[M′(CN)6] (M′ = Fe, Co) result in the formation of CN-bridged coordination polymers. The analysis of the products obtained suggests that they have a two-dimensional structure, in which ditopic [M3(dpa)4]2+ linkers bridge 4-connected [M′(CN)6]3– nodes into an extended layer. The synthesis of {[Co3(dpa)4]1.97[Fe(CN)6]}Cl0.8 (1) is accompanied by an electron transfer from the tricobalt to the hexacyanoferrate units, which results in the formation of [Co3(dpa)4]3+ and [Fe(CN)6]4– fragments. In {[Ni3(dpa)4]1.74[Fe(CN)6]}Cl0.45 (3), a partial charge transfer between the trinickel and the hexacyanoferrate units leads to the temperature-dependent FeIII/FeII mixed valence, and lower temperatures favor the thermodynamic FeIII ground state. {[Co3(dpa)4]2.06[Co(CN)6]}Cl1.1 (2) exhibits spin-glass behavior with a spin-freezing point of approximately 4.8 K, which is due to the magnetic superexchange between the paramagnetic [Co3(dpa)4]2+ (S = 1/2) units through the diamagnetic [Co(CN)6]3– linkers.

LaCo2As2 single crystals of good quality are prepared from a 1:2:2 mixture of La, Co, and As in a Bi flux (evacuated quartz tube, 1173 K, 10 d).

LaCo(2)As(2) can be synthesized as pure crystalline material by annealing a mixture of elements in Bi flux. The reaction, however, is accompanied by the incorporation of a small quantity of Bi into the structure and the formation of vacancies in the Co sublattice, which lead to substantial changes in structural and magnetic properties of the material.

B-site ordered double perovskites of the type AA′BB′O6 have attracted attention recently due to unusual and unpredictable modes of magnetic order. Of particular interest are materials where AA′ = Ba, Sr or La and B′ = Mo,Ru,Os and Re and B is a diamagnetic ion such as Li+,Na+,Ca2+, Mg2+, Zn2+ or Cd2+.Crystal symmetries encountered are Fm-3m, I4/m and P21/n as a function of the Goldschmidt tolerance factor which in turn impose local symmetries at the magnetic B′ site of m-3m, 4/m and -1, respectively. The magnetic ions have electronic configurations 4d1, 4d3,5d1, 5d2 and 5d3 with nominal spins, S = 1/2,1 and 3/2. Spin orbit coupling is very large, reaching ~ 0.5eV for the 5d ions, and plays a more important role than in the more familiar 3d-based magnetic materials. Geometric frustration, the B′ ion sublattice is f.c.c., based on edge sharing tetrahedra, also is of critical importance. Observed magnetic ground states range from spin glasses (without apparent disorder), collective spin singlets, ferromagnets and antiferromagnets. Often, ordered moments are a small fraction of a Bohr magneton. Systematic behavior is elusive. For example Ba2YReO6 and Ba2CaOsO6 (both Fm-3m) have the same unit cell constant to within .01 % and both are based on 5d2 ions but the former is a highly unconventional spin glass and the latter orders antiferromagnetically below 50K. Recent results on this class of magnetic materials will be described in the context of existing theories.

Quaternary phases La1−xNdxCo2P2 (x = 0, 0.12, 0.25, 0.37, 0.50, 0.63, 0.75, 0.88, 1.0) have been synthesized from Sn flux to investigate the origins of drastic differences in properties between ferromagnetic LaCo2P2 and antiferromagnetic NdCo2P2. Powder and single-crystal X-ray diffraction indicate that all La1−xNdxCo2P2 samples are isostructural and crystallize in the ThCr2Si2 structure type. The unit cell parameters and volume change non-linearly with the Nd content (x), with the x 0.50 being closer to NdCo2P2. These structural differences are also reflected in the magnetic behavior. The samples with lower Nd content are characterized by ferromagnetic ordering in the Co sublattice with the TC increasing from 132 K for x = 0 to 262 K for x = 0.50, while the samples with higher Nd content exhibit suppressed magnetization in the Co sublattice and canted antiferromagnetic ordering with TC ∼ 270 K. Refinement of neutron powder diffraction patterns for x = 0.50 and 0.75 reveals a gradual ordering of the Nd 4f moments under the influence of Co 3d moments below 100 K. At low temperatures and zero field, these samples exhibit antiferromagnetic ordering of both Nd and Co magnetic moments, but under applied field they demonstrate the stabilization of a ferrimagnetic state with antiparallel alignment of the 4f and 3d moments, as indicated by isothermal magnetization measurements. The re-entrant ferrimagnetic transition is also observed in samples with x > 0.50 if the temperature is lowered below 5 K. The occurrence of this low-temperature magnetic transition was confirmed by alternating-current susceptibility measurements.

Journal of physics / Conference Series 430(conference 1), 012105(2013). doi:10.1088/1742-6596/430/1/012105, A detailed combined study of local electronic and crystal structures was performed for a series of rare-earth cobalt phosphides RCo2P2 (R = La, Ce, Pr, Nd, Eu) with peculiar itinerant magnetic ordering schemes using different methods employing the synchrotron radiation: RIXS, XANES and EXAFS. Europium and cerium are shown to be in the intermediate valence state in all compounds. The correlation between rare-earth valence and the observed changes of magnetic ordering in the systems under investigation is discussed., Published by IOP Publ., Bristol

The local structure of the defect, vacancy disordered fluorite Yb3TaO7 has been investigated using neutron total scattering methods. The average structure of Yb3TaO7 is well described in space group Fmm, with unit cell constant a = 5.1872(1) A, in which Yb and Ta occupy, randomly, the single cation site in a fluorite structure and there is a vacancy concentration of approximately ⅛ on the anion site. Examination of the neutron diffraction data shows many features in the background which can be ascribed to local order of Yb, Ta, and O atoms. The pair distribution function, G(r), was analyzed using both crystallographic models and Reverse Monte Carlo (RMC) methods. From both perspectives the local structure resembles very strongly the cation and vacancy ordered model expected by extrapolation from the structure of Ho3TaO7, space group C2221. As well, this local structure model provides a superior fit to the G(r) out to ∼30 A compared to the average structure model. This result may have implications for the understanding of phenomena such as catalytic activity which are reported to depend on order/disorder structural transformations in the series, RE3TaO7 and RE3NbO7 where RE is a rare earth element.

To clarify the origins of different magnetic behavior in ferromagnetic LaCo2P2 and antiferromagnetic RCo2P2 (R=Ce, Pr, Nd, and Sm), quaternary phases La0.9R0.1Co2P2 have been investigated. While LaCo2P2 orders ferromagnetically at 132 K, the introduction of the small amount of R into the La sublattice increases the ferromagnetic ordering temperature, which correlates with the size of the R3+ ion (TC=144 K, 150 K, 161 K, and 165 K for Ce, Pr, Nd, and Sm, respectively). This change is attributed to the increasing Co–Co separation within the [Co2P2] layer, which results in higher spin polarization of the Co 3d subband. The low-temperature magnetism of La0.9R0.1Co2P2 strongly depends on the nature of the rare-earth metal.