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Low-temperature reaction of A-site-ordered layered perovskite Eu

The Co{sub 2}(OH)(PO{sub 4}){sub 1-x}(AsO{sub 4}){sub x} [0{

Compounds of the general formula Co2?xNix(OH)PO4 (x = 0.1, 0.3) have been synthesized under mild hydrothermal conditions. Neutron powder diffraction, susceptibility and heat capacity measurements were carried out on polycrystalline samples. The cobalt?nickel compounds are ordered as three-dimensional antiferromagnets with ordering temperatures of 70 and 64?K for x = 0.1 and x = 0.3, respectively. The magnetic study shows a spin glass-like state below 11 and 5?K for Co1.9Ni0.1(OH)PO4 and Co1.7Ni0.3(OH)PO4, respectively. Specific heat data present peaks at 68 and 61?K for Co1.9Ni0.1 and Co1.7Ni0.3, respectively. These peaks show broad shoulders between approximately 15 and 40?K. The lack of any distinguishable anomaly below 10?K supports the spin glass nature of the low temperature transitions. Refinement of room temperature neutron diffraction data indicates that the Ni(II) ions are in octahedral co-ordination with the practical absence of these ions in the trigonal bipyramidal sites. The magnetic structures of Co2?xNix(OH)PO4 consist of ferromagnetic arrangements between the octahedral chains and trigonal bipyramidal dimers within the xz plane with the magnetic moments along the z axis. The ferromagnetic layers are disposed antiparallel to one another along the y direction establishing the three-dimensional antiferromagnetic order (TN?70?K for Co1.9Ni0.1 and ?64?K for Co1.7Ni0.3). The different exchange pathways, the anisotropy of the Co(II) ions and the frustration of the magnetic moments in the trigonal bipyramidal geometry could be responsible for the freezing process.

An amorphous GdNiAl sample was prepared by mechanical grinding performed on a crystallised intermetallic. The treatment changes greatly the ferromagnetic behaviour of this compound; its Curie temperature decreases from 57 K (unmilled sample) to 29 K (milled sample). Specific heat and magnetisation measurements reveal that amorphous GdNiAl exhibits an interesting magnetocaloric effect; for an applied magnetic field of 5 T a change of ≅8.9 J/kg K is observed at 36 K for the isothermal magnetic entropy.

The hydroxyphosphates with formula Co1.7M0.3(OH)PO4 (M = Ni, Cu) have been prepared from hydrothermal synthesis. The compounds have been characterized by X-ray powder diffraction and spectroscopic measurements. Diffuse reflectance data of both phases show bands belonging to the two chromophores, octahedral and trigonal bipyramidal, of the Co(II) ions together with others associated to the chromophores of nickel(II) and copper(II) ions. Magnetization measurements of Co1.7Ni0.3(OH)PO4 show the presence of two maxima at ca. 62 and 5 K, respectively. The first peak was attributed to a three-dimensional antiferromagnetic ordering and the second one reveals the existence of a spin glass-like state. This behaviour was confirmed from the ac measurements obtained at different frequencies and applied fields. The Co1.7Cu0.3(OH)PO4 phase exhibits only one maximum at 65 K in both χm and ac measurements associated to the three-dimensional antiferromagnetic interactions; however, the existence of spin glass behaviour is not observed. ZFC-FC curves for the cobalt–nickel and cobalt–copper compounds show irreversibility just below TN indicating the existence of higher spin decompensations than in the Co2(OH)PO4 phase. This difference and its influence in the magnetic anomalies observed at low temperatures is the more important effect in the magnetic measurements of the ordered Co2(OH)PO4 phase substituted at about 15% of the Co2+ (S = ) by Ni2+ (S = 1) or Cu2+ (S = ½) ions. The origin of the frustration, which is necessary to achieve a disordered ground state, is investigated in these kinds of spin glass compounds. Taking into account the possible magnetic exchange pathways, which are essential to install competition and ensure cooperativeness of the freezing process, correlations between structural and magnetic properties have been performed.

We present electrical resistivity measurements on U 1− x La x Pt ( x =0, 0.1, 0.2, 0.5, 0.8, 0.9, 1) between 2 and 1000 K. The compounds exhibiting ferromagnetic behaviour ( x ≤0.3) display a maximum at low temperatures and some of them a minimum at higher temperatures. For the non-ordered compounds ( x ≥0.5) the resistivity increases continuously with temperature. A common feature of the U-rich compounds is the existence of a negative logarithmic temperature dependence of the total resistivity above the maximum, whereas a combination of a linear term plus a logarithmic term is found for the others. Under a Kondo-like framework, a breakdown of the Mattiessen rule is suggested.

Measurements of the magnetic, thermal, and transport properties of the ${\text{CeIn}}_{2}$ binary alloy are consistent with a paramagnetic-ferromagnetic transition at ${T}_{C}=22\text{ }\text{K}$. A discontinuity in the magnetic entropy, electrical resistivity and thermal expansion, and a huge anomaly in the specific heat of 113 J/mol K $(\ensuremath{\Delta}{c}_{mag}=103\text{ }\text{J}/\text{mol}\text{ }\text{K})$, at the magnetic transition, are observed. In addition, the Arrott plots show negative slope at low fields, the field-cooling and field-warming magnetization present irreversibility, and both the susceptibility and the resistivity evidence a small thermal hysteresis of 0.05 K. Moreover, the values of the entropy change calculated from the magnetization data using the Clausius-Clapeyron equation are in good agreement with those directly obtained from the specific-heat data. The joint analysis of all these results provides evidence for the first order character of this magnetic transition in ${\text{CeIn}}_{2}$.

Hydrogenation of CeNiSn provokes a change of the ground state of cerium from Kondo insulator to antiferromagnetic ( CeNiSnH 1.0 ) or ferromagnetic ( CeNiSnH 1.8 ) behaviour. We report heat capacity measurements in CeNiSnH 1.0 performed in magnetic fields up to 90 kOe. The calculated magnetocaloric effect has a maximum at temperatures close to T N = 4.5 K . The values of the isothermal magnetic entropy ( Δ S m ) and the adiabatic temperature change ( Δ T ad ) are modest, and similar to those found in other Ce compounds. The field dependence of both, heat capacity and magnetocaloric effect clearly indicate the nature of the magnetic transition.

The ternary compounds CeCoSi and CeCoGe exhibit an antiferromagnetic ordering at 8.8 and 5.0 K, respectively. We present magnetization and specific heat measurements on these compounds and their hydrides, CeCoSiH and CeCoGeH. The hydrogen absorption reveals an original transition from antiferromagnetic to spin fluctuation behavior. CeCoSiH and CeCoGeH present a spin fluctuation temperature T sf around 130 and 15 K, respectively.

We have studied both the crystal chemistry and magnetic, transport, and thermal properties of the hydride $\mathrm{Ce}\mathrm{Ni}\mathrm{Ga}{\mathrm{H}}_{1.1(1)}$. This compound crystallizes in the hexagonal $\mathrm{Al}{\mathrm{B}}_{2}$-type structure with a random distribution of nickel and gallium atoms on the B site, which has an important influence upon the macroscopic properties. Its thermoelectric power versus temperature indicates that cerium is in a trivalent state. The electrical resistivity displays two minima, which could be expected for the Kondo-type interactions in the presence of crystal field effects. Specific heat measurements up to $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ allow us to determine the splitting energies ${\ensuremath{\Delta}}_{1}=100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${\ensuremath{\Delta}}_{2}=159\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A broad maximum is observed around $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the specific heat in the low temperature region. This maximum and its evolution with the applied magnetic field, are discussed in the framework of the existing theories, which point toward the existence of short-range magnetic correlations and spin glasslike freezing below $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This study reveals: (i) that the hydrogenation of the intermediate valence gallide CeNiGa induces a valence transition for cerium which is purely trivalent in the hydride and (ii) the absence above $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ of long-range magnetic ordering resulting from structural disorder around Ce atoms.

The hydrogenation process of the intermediate valence compounds CeNiAl, CeNiGa, CeNiIn, CeNiSi and CeNiGe and of the Kondo semiconductor stannide CeNiSn has been investigated. Magnetization and electrical resistivity measurements reveal interesting physical transition induced by the insertion of hydrogen: (i) the Kondo temperature decreases from 600(20) to 220(10) K in the sequence CeNiGe → CeNiGeH1.6(1); (ii) the hydrides CeNiAlH1.93(5) and CeNiGaH1.1(1) contain cerium in trivalent state and the former shows the presence of magnetic correlations around 3.6(2) K; (iii) in contrast, CeNiInH1.8(2) presents a ferromagnetic transition at TC=6.8(2) K and a magnetic transition Kondo semiconductor → antiferromagnet (TN=4.5(2) K) → ferromagnet (TC=7.0(2) K) is evidenced in the CeNiSnHY system. These results are explained by a modification of the competition between RKKY magnetic interaction and Kondo interaction induced by the hydrogenation of these intermetallics.

The new hydrides CeNiSnH 1.0(2) and CeNiSnH 1.8(2) have been investigated by X-ray powder diffraction, magnetization, electrical resistivity and specific heat measurements. The hydrogen insertion into CeNiSn reveals an interesting magnetic transition: Kondo insulator (CeNiSn)→antiferromagnet (CeNiSnH 1.0(2) )→ferromagnet (CeNiSnH 1.8(2) ). The physical properties of these compounds are influenced by the Kondo effect.

The Laves phases GdMn 2 has been recently prepared in amorphous structure. We present heat capacity measurements under magnetic fields up to 9 T in both polycrystalline and amorphous states. No distinguishable anomaly at the magnetic order temperature is observed in the amorphous sample. In both cases a very low field dependence of the heat capacity is observed. The calculated magnetocaloric effects give rise to a maximum centred at 37 K in the crystalline form and a low, but broad maximum centred at 130 K in the amorphous sample.

In the intermediate valence (IV) CeNiIn, the Ce ions evolve towards a trivalent state with a ferromagnetic transition at low temperature when hydrogen is interstitially inserted. In this work, we present heat capacity measurements on CeNiInH 1.8 carried out under magnetic fields up to H = 90 kOe . The observed λ -anomaly and its evolution with magnetic field confirm the existence of the ferromagnetic transition. The total entropy shifts to higher temperatures with increasing field, giving rise to the appearance of magnetocaloric effect. We have found that both the isothermal magnetic entropy ( Δ S m ) and the adiabatic temperature change ( Δ T ad ) display significant peaks around 6.5 K, despite the small magnetic moment of the Ce ions.

The isostructural Co 2− x Cu x (OH)PO 4 ( x =0, 1 and 2) phases have been prepared from hydrothermal synthesis and characterized from powder X-ray diffraction. The structure consists of a three-dimensional framework in which M(1)O 5 -trigonal bipyramid dimers and M(2)O 6 -octahedral chains are simultaneously present. Magnetization measurements of Co 2 (OH)(PO 4 ) show the existence of two maxima attributed to a three-dimensional antiferromagnetic ordering at 70 K and a spin-glass-like state at 12 K. When Co 2+ is substituted by Cu 2+ ions, the spin-glass behavior disappears and the magnetic order is decreased.