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Cd0.9Zn0.1Te crystals were grown using a modified vertical Bridgman growth technique to produce radiation detector grade material. Motivated by the importance of the retrograde solubility problem in II-VI compound semiconductors, particularly CZT, different cool down techniques were used to observe the effects on the secondary phase (SP) size, distribution, density and resulting detector properties. Presented are four of the cool down schemes performed and the results in terms of the secondary phases and charge transport characteristics of the grown crystals. The cooling of the grown ingot to ambient temperatures was done over various lengths of time. The differences between the cool down methods are in the rates of cooling through the retrograde solubility phase. Apart from these cool downs, quenching studies were also performed on some crystal growths giving unique results in terms of the secondary phase distribution and characteristics as radiation detectors. [ABSTRACT FROM AUTHOR]

Abstract: Crystal growth in the ZnO–TeO2 system was investigated using Czochralski technique in a 2.5kHz induction heating system. Several runs and experiments helped optimize the Zn2Te3O8 growth process, which was limited by quite a few difficulties. These difficulties include the evaporation of TeO2 material above 700°C, the formation of more than one phase during the growth, and the lack of a Zn2Te3O8 single crystal to initiate the growth. The main and most persisting problem is that there is no stable phase in the system that forms a line compound at which the crystal growth should be attempted. The resulting material was formed of many single crystals and a mixture of other phases. Single Zn2Te3O8 crystals of sizes ranging between 50 and 200mm3 resulted when the pulling rate was 1.1mm/h and the rotation speed was 12rpm. These single crystals were extracted and their optical and electrical properties were studied for the first time. Using other pulling rates and rotation speeds returned smaller crystals with sizes ranging between 15 and 35mm3. [Copyright &y& Elsevier]

We report polarization dependent photoluminescence studies on unintentionally-, Mg-, and Ca-doped β-Ga2O3 bulk crystals grown by the Czochralski method. In particular, we observe a wavelength shift of the highest-energy UV emission which is dependent on the pump photon energy and polarization. For 240 nm (5.17 eV) excitation almost no shift of the UV emission is observed between E||b and E||c, while a shift of the UV emission centroid is clearly observed for 266 nm (4.66 eV), a photon energy lying between the band absorption onsets for the two polarizations. These results are consistent with UV emission originating from transitions between conduction band electrons and two differentially-populated self-trapped hole (STH) states. Calcuations based on hybrid and self-interaction-corrected density functional theories further validate that the polarization dependence is consistent with the relative stability of two STHs. This observation implies that the STHs form primarily at the oxygen atoms involved in the original photon absorption event, thus providing the connection between incident polarization and emission wavelength. The data imposes a lower bound on the energy separation between the self-trapped hole states of ~70-160 meV, which is supported by the calculations. [ABSTRACT FROM AUTHOR]

Cadmium telluride (CdTe) high purity, bulk, crystal ingots doped with phosphorus were grown by the vertical Bridgman melt growth technique to understand and improve dopant solubility and activation. Large net carrier densities have been reproducibly obtained from as-grown ingots, indicating successful incorporation of dopants into the lattice. However, net carrier density values are orders of magnitude lower than the solubility of P in CdTe as reported in literature, 1018/cm3 to 1019/cm3 [J. H. Greenberg, J. Cryst. Growth 161, 1–11 (1996) and R. B. Hall and H. H. Woodbury, J. Appl. Phys. 39(12), 5361–5365 (1968)], despite comparable starting charge dopant densities. Growth conditions, such as melt stoichiometry and post growth cooling, are shown to have significant impacts on dopant solubility. This study demonstrates that a significant portion of the dopant becomes incorporated into second phase defects as compounds of cadmium and phosphorous, such as cadmium phosphide, which inhibits dopant incorporation into the lattice and limits maximum attainable net carrier density in bulk crystals. Here, we present an extensive study on the characteristics of these second phase defects in relation to their composition and formation kinetics while providing a pathway to minimize their formation and enhance solubility. [ABSTRACT FROM AUTHOR]

We applied time-resolved pump-probe spectroscopy based on free carrier absorption and light diffraction on a transient grating for direct measurements of the carrier lifetime and diffusion coefficient D in high-resistivity single crystal CdTe (codoped with In and Er). The bulk carrier lifetime τ decreased from 670 ± 50 ns to 60 ± 10 ns with increase of excess carrier density N from 1016 to 5 x 1018 cm-3 due to the excitation-dependent radiative recombination rate. In this N range, the carrier diffusion length dropped from 14 µm to 6 µm due to lifetime decrease. Modeling of in-depth (axial) and in-plane (lateral) carrier diffusion provided the value of surface recombination velocity S = 6 x 105 cm/s for the untreated surface. At even higher excitations, in the 1019-3 x 1020 cm-3 density range, D increase from 5 to 20 cm²/s due to carrier degeneracy was observed. [ABSTRACT FROM AUTHOR]

Cadmium telluride (CdTe) semiconductors are used in thin-film photovoltaics, detectors, and other optoelectronic applications. For all technologies, higher efficiency and sensitivity are achieved with reduced charge carrier recombination. In this study, we use state-of-the-art CdTe single crystals and electro-optical measurements to develop a detailed understanding of recombination rate dependence on excitation and temperature in CdTe. We study recombination and carrier dynamics in high-resistivity (undoped) and arsenic (As)-doped CdTe by employing absorption, the Hall effect, time-resolved photoluminescence, and pump-probe in the 80–600 K temperature range. We report extraordinarily long lifetimes (30 µs) at low temperatures in bulk undoped CdTe. Temperature dependencies of carrier density and mobility reveal ionization of the main acceptors and donors as well as dominant scattering by ionized impurities. We also distinguish different recombination defects. In particular, shallow AsTe and deep VCd−AsCd acceptors were responsible for p-type conductivity. AX donors were responsible for electron capture, while nonradiative recombination centers (VCd−AsTe, As2 precipitates), and native defects (VCd−TeCd) were found to be dominant in p-type and n-type CdTe, respectively. Bimolecular and surface recombination rate temperature dependencies were also revealed, with bimolecular coefficient T−3/2 temperature dependence and 170 meV effective surface barrier, leading to an increase in surface recombination velocity at high temperatures and excitations. The results of this study allowed us to conclude that enhanced crucible rotation growth of As-doped CdTe is advantageous to As activation, leading to longer lifetimes and larger mobilities and open-circuit voltages due to lower absorption and trapping. [ABSTRACT FROM AUTHOR]

Abstract: A modified high-pressure vertical Bridgman (HPVB) method has been used to grow single crystals of lead magnesium niobate–lead titanate (PMN–PT) using self-seeding. The starting charge was prepared close to stoichiometric composition of (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3, ranging from x=0.29 to x=0.32. The ingots grown had a top diameter of ∼114mm and weighing ∼1.9kg with a volume of ∼230cc. Standard form off-the-shelf platinum crucibles from vendors with a wall thickness ∼0.34±0.01mm were used. The thinner platinum helped to attain a larger radial thermal gradient as well as enhancing the cost effectiveness of the process by reducing usage of precious metals. An oxygen partial pressure of 8000–10000ppm was used and controlled during the entire growth process. Self-seeding technique was used in order to attempt to grow large diameter single crystals of PMN–PT. The composition variation in the ingot is dependent on the growth rate, thermal gradients and oxygen partial pressure. The segregation coefficient of Ti was 0.86±0.008 for the starting composition of x=0.32. [Copyright &y& Elsevier]

Performing deep level transient spectroscopy (DLTS) on Schottky diodes, we investigated defect levels below the conduction band minima (Ec) in Czochralski-grown unintentionally doped (UID) and vertical gradient freeze-grown Zr-doped β-Ga2O3 crystals. In UID crystals with an electron concentration of 1017 cm−3, we observe levels at 0.18 eV and 0.46 eV in addition to the previously reported 0.86 (E2) and 1.03 eV (E3) levels. For 1018 cm−3 Zr-doped Ga2O3, signatures at 0.30 eV (E15) and 0.71 eV (E16) are present. For the highest Zr doping of 5 × 1018 cm−3, we observe only one signature at 0.59 eV. Electric field-enhanced emission rates are demonstrated via increasing the reverse bias during measurement. The 0.86 eV signature in the UID sample displays phonon-assisted tunneling enhanced thermal emission and is consistent with the widely reported E2 (FeGa) defect. The 0.71 eV (E16) signature in the lower-Zr-doped crystal also exhibits phonon-assisted tunneling emission enhancement. Taking into account that the high doping in the Zr-doped diodes also increases the electric field, we propose that the 0.59 eV signature in the highest Zr-doped sample likely corresponds to the 0.71 eV signature in lower-doped samples. Our analysis highlights the importance of testing for and reporting on field-enhanced emission, especially the electric field present during DLTS and other characterization experiments on β-Ga2O3 along with the standard emission energy, cross section, and lambda-corrected trap density. This is important because of the intended use of β-Ga2O3 in high-field devices and the many orders of magnitude of possible doping. [ABSTRACT FROM AUTHOR]

Currently, Fe doping in the ∼1018 cm−3 range is the most widely available method for producing semi-insulating single crystalline β -Ga2O3 substrates. Red luminescence features have been reported from multiple types of Ga2O3 samples, including Fe-doped β -Ga2O3, and attributed to Fe or NO. Herein, however, we demonstrate that the high-intensity red luminescence from Fe-doped β-Ga2O3 commercial substrates consisting of two sharp peaks at 689 nm and 697 nm superimposed on a broader peak centered at 710 nm originates from Cr impurities present at a concentration near 2 ppm. The red emission exhibiting a twofold symmetry, peaks in intensity for excitation near the absorption edge, seems to compete with the Ga2O3 emission at a higher excitation energy and appears to be intensified in the presence of Fe. Based on the polarized absorption, luminescence observations, and the Tanabe–Sugano diagram analysis, we propose a resonant energy transfer of photogenerated carriers in the β -Ga2O3 matrix to octahedrally coordinated Cr3+ to give red luminescence, possibly also sensitized by Fe3+. [ABSTRACT FROM AUTHOR]

Highlights • This is the first time that low-energy implanted positrons studies are exclusively performed for the systems studied here. • This is also the first time that a measurement of 3D X-ray imaging has been applied to this type of samples. • We report here a ferrite system prepared in a non-standard method. Abstract Samples of Nickel-Zinc ferrite, Ni 1− x Zn x Fe 2 O 4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0), were synthesized by a solid state reaction method. The morphology of four of the samples (x = 0.0, 0.4, 0.6 and 1.0) was observed with 3D X-ray imaging. The distortion caused by the increase of zinc in the ferrite structure and the respective movement of the trivalent (Fe) and divalent (Ni) ions from the tetrahedral- to the octahedral-sites cause some vacancies which act as positron traps. Doppler broadening of annihilation radiation experiments have been performed with a variable energy monoenergetic positron beam. The positron results as a function of the energy/depth implantation are presented in this work. [ABSTRACT FROM AUTHOR]

Highlights • Improved light yield and scintillation decay time with calcium co-doping. • Presence of Ce(IV) caused by calcium co-doping. • Over order of magnitude increase in the concentration of vacancies. • Competition of Ce(IV) luminescence and non-radiative mechanism. Abstract Co-doping with divalent elements is known to improve the light yield and decay times of some cerium-activated scintillators, despite the stabilization of tetravalent Ce4+, previously believed to be non-luminescent, from the Ce3+ state. Ce4+ stabilization is a charge compensation mechanism which results from divalent ion substitution of a 3 + site. To elucidate the underlying mechanisms, which remain poorly understood, we have grown three Ce,Ca:YAG crystals with different amounts of calcium co-dopants by the Czochralski method and characterized their scintillation and defect properties. Calcium co-doping reduces the decay times and stabilizes the formation of Ce4+ as expected. Interestingly, X-ray fluorescence analysis reveals a decreased concentration of cerium within the YAG crystal for the sample doped with high levels of Ca, which could contribute to the observation that scintillator properties are improved only for low levels of Ca co-doping. Additionally, positron annihilation spectroscopy reveals an increase in the concentration of vacancies with increasing Ca concentration, while thermoluminescence is observed to show no detectable signal. Furthermore, room temperature photoluminescence of the Ce 4f to 5d1 transition demonstrates decreased emission with increasing Ca co-doping. These data suggest that Ca co-doping decreases the decay time by creating defects with non-radiative decay pathways. [ABSTRACT FROM AUTHOR]

Magnesium-doped gallium oxide may be utilized as a semi-insulating material for future generations of power devices. Spectroscopy and hybrid functional calculations were used to investigate defect levels in Czochralski-grown β-Ga2O3. Substitutional Mg dopants act as deep acceptors, while substitutional Ir impurities are deep donors. Hydrogen-annealed Ga2O3:Mg shows an IR peak at 3492 cm−1, assigned to an O-H bond-stretching mode of a neutral MgH complex. Despite compensation by Ir and Si and hydrogen passivation, high concentrations of Mg (1019 cm−3) can push the Fermi level to mid-gap or lower. [ABSTRACT FROM AUTHOR]

Research is described here that is directed toward obtaining p -type ZnO crystals either by doping or by creating native defects. Theoretically, zinc vacancies are shallow acceptors that should allow for p -type conduction. Bulk ZnO crystals can be grown by chemical vapor transport (CVT), melt solidification, and hydrothermal methods. Here we have explored annealing processes with the goal of creating zinc vacancies in bulk ZnO crystals. Positron annihilation spectra reveal the reproducible formation of high concentrations (>10 20 cm −3 ) of isolated zinc vacancies ( V Zn ) in oxygen-annealed, CVT-grown ZnO crystals in the ∼100–150 nm near-surface. Melt- and hydrothermally grown samples, in contrast, show insignificant levels of zinc vacancy creation. Photoluminescence (PL) emission spectra indicate a V Zn (0/-1) acceptor level at ∼ 155–165 meV; red PL (∼1.7 eV) emission, related to the V Zn (-1/-2) level, was also observed. Infrared absorption spectroscopy reveals the presence of a zinc vacancy complex with a hole binding energy range of 420–450 meV – and with a continuum suggesting a p -type region. XPS measurements support the deficiency of Zn after oxygen annealing the CVT-grown ZnO single crystal. [ABSTRACT FROM AUTHOR]

In this study, LiAlO 2 crystals doped with rare-earth elements and Ti were produced by the CZ method and spectroscopic and neutron detection properties were investigated. Photoluminescence revealed no clear luminescent activation of LiAlO 2 by the rare-earth dopants though some interesting luminescence was observed from secondary phases within the crystal. Gamma-ray pulse height spectra collected using a 137 Cs source exhibited only a Compton edge for the crystals. Neutron modeling using Monte Carlo N-Particle Transport Code revealed most neutrons used in the detection setup are thermalized, and while using natural lithium in the crystal growth, which contains 7.6% 6 Li, a 10 mm Ø by 10 mm sample of LiAlO 2 has a 70.7% intrinsic thermal neutron capture efficiency. Furthermore, the pulse height spectra collected using a 241 Am-Be neutron source demonstrated a distinct neutron peak. [ABSTRACT FROM AUTHOR]

The storage capacity of positrons in micro-Penning-Malmberg traps with large length to radius aspect ratios and radii of tens of microns was explored. Simulation studies were conducted with the WARP code and Charged Particle Optics program. A new design of the Penning-Malmberg trap consisting of an array of microtraps with substantially lower end electrode potential than conventional traps was considered. Simulations demonstrated each microtrap with 50 μm radius immersed in a 7T uniform magnetic field could store positrons indefinitely with a density of 1.6 × 1011cm-3 while the confinement voltage was only 10V. For microtraps with radii between 100 μm and 3 μm, the particle density scaled as r-2. Charge clouds developed the expected radial density distribution (that of a soft edge) and rigid rotation evolved to some extent. Plasma confinement time was independent of trap length. [ABSTRACT FROM AUTHOR]

Abstract: A combination of atomic force microscopy, optical microscopy, and mass spectrometry was employed to study CdZnTe crystal surface and used etchant solution following exposure of the CdZnTe crystal to the Everson etch solution. We discuss the results of these studies in relationship to the initial surface preparation methods, the performance of the crystals as radiation spectrometers, the observed etch pit densities, and the chemical mechanism of surface etching. Our results show that the surface features that are exposed to etchants result from interactions with the chemical components of the etchants as well as pre-existing mechanical polishing. [Copyright &y& Elsevier]

Abstract: A transient, coupled model has been developed to analyze the segregation of zinc in cadmium zinc telluride (CZT) grown in an electrodynamic gradient freeze (EDG) furnace. The coupled model consists of a local model that solves for time-dependent melt flow, heat transfer, melt–crystal interface position, and zinc distribution in both melt and solid phases and a quasi-steady-state global model that features realistic furnace heat transfer. After verification and validation tests, the model is applied to predict composition patterns in a large-scale CZT EDG growth system previously analyzed by Gasperino et al. [On crucible effects during the growth of cadmium zinc telluride in an electrodynamic gradient freeze furnace, J. Crys. Growth 311 (2009) 2327–2335]. Surprisingly, anomalous zinc segregation is predicted, featuring a non-monotonic axial concentration profile and several local minima and maxima across the boule. A mechanistic explanation is put forth based on the cumulative effect of changes in multi-cellular melt flow structures, a particularly susceptible occurrence for CZT systems. Additional effects of furnace translation rate and solid state diffusion are probed. [Copyright &y& Elsevier]

Single crystals of Nd:YVO4 grown with the Czochralski technique frequently exhibit light scattering defects that are detrimental to their lasing and optical properties. Defects in the form of low angle grain boundaries have been characterized in what are nominally ‘single crystals’. The misorientation angles of the boundaries were determined to be typically < 1°, which corresponds to formation energies of approximately 1 Jm−2. It was found that dislocations generated during crystal growth and cooling have enough mobility in certain growth directions to form low angle grain boundaries through polygonization. Despite the relatively high energies the boundaries were stable, being immobile at annealing temperatures up to 97% of the melting point (2083 K). Suggestions are made to reduce or eliminate polygonization, including the addition of atoms with a size either much larger or smaller than Y3+. [ABSTRACT FROM AUTHOR]

Abstract: High-resistivity crystals of CdZnTe are made using low-pressure Bridgman. The success of any doping scheme depends on its effectiveness in overcoming trace contamination and variations in processing. Multiple imaging and quantitative techniques gave highly complimentary information relating to spatial heterogeneity of the cadmium zinc telluride (CZT). Secondary Ion Mass Spectroscopy (SIMS) has good sensitivity and can quantitate trace elements in areas less than 150μm in diameter. PNNL has SIMS standards for several key elements and can extrapolate for other elements. A heavily Pb-doped sample (0.1atm%) was characterized to determine the distribution of lead and its potential impact on precipitates. [Copyright &y& Elsevier]

• Inclusion distributions improved with increased tellurium concentrations. • Highly excess tellurium assists in forced melt convection. • Material was produced at competitive growth rates without the need for post growth thermal processing. • Flux inclusion distributions appropriate for thick detector performance were achieved. The presence of second phase defects, particularly flux inclusions of tellurium rich composition, are of great concern for charge collection efficiency in cadmium zinc telluride (CZT) and cadmium telluride (CT) material intended for applications such as radiation detection. These inclusions can distort applied electric field lines within the detector as well as act as trapping centers for charge carriers. Reduction and/or elimination of these inclusions is required to achieve appropriate charge collection efficiencies, especially in detectors of thicknesses greater than 5 mm. These so-called flux inclusions are understood to form as a consequence of constitutional undercooling at the crystal growth interface. In this study, a forced melt convection technique was applied in Vertical Bridgman (VB) melt growth of CZT without reducing imposed growth rates of ∼2 mm/hr. Several rotation profiles were tested while adjusting the melt composition from 51.62 to 61.75 atomic percent (at%) Te where the Te concentration was initially increased to improve overall material purity. With forced melt convection, the best inclusion distributions were achieved with highly Te rich melt compositions, far beyond the stoichiometric composition range for the CZT system. Average inclusion diameters were reduced to 2 µm while inclusions greater than 5 µm were essentially eliminated. Composition analyses of these ingots revealed near equilibrium concentrations of Te, even with ingots grown from Te concentrations as high as 61.75 at% Te. In this paper, a recipe for the reduction of inclusions in CZT melt growth is put forward and the implications of this method on our understanding of inclusion formation are discussed. [ABSTRACT FROM AUTHOR]

• High pressure Bridgman technique is suitable to synthesize CdTe from elemental sources. • Highly volatile dopants can be efficiently incorporated into melt grown CdTe under high pressure. • High purity level in CdTe is achievable in scalable HPB melt growth process. Efficient, safe and cost-effective synthesis of CdTe from elements is rather challenging in silica sealed ampoules due to the high vapor pressure of Cd. In this article, we report on the integrated synthesis and crystal growth of high-purity CdTe using the high pressure Bridgman (HPB) technique that is scalable to large volumes. The process lends itself for cost competitive industrial production of polycrystalline feedstock material for photovoltaics, sensors and electro-optic applications. Cadmium telluride (CdTe) crystals exceeding 1 kg in size were synthesized from elemental Cd and Te sources with purity comparable to state-of-the-art gamma ray detector crystals. In addition, synthesis of highly-doped CdTe feedstock for thin film photovoltaics applications demonstrating effective incorporation of group V (As, Sb) dopants was achieved at growth speeds of ~500 mm/hr. The technique may be applicable to produce other II-VI compounds with volatile components. [ABSTRACT FROM AUTHOR]

We report polarization dependent photoluminescence studies on unintentionally-, Mg-, and Ca-doped β-Ga 2 O 3 bulk crystals grown by the Czochralski method. In particular, we observe a wavelength shift of the highest-energy UV emission which is dependent on the pump photon energy and polarization. For 240 nm (5.17 eV) excitation almost no shift of the UV emission is observed between E||b and E||c, while a shift of the UV emission centroid is clearly observed for 266 nm (4.66 eV), a photon energy lying between the band absorption onsets for the two polarizations. These results are consistent with UV emission originating from transitions between conduction band electrons and two differentially-populated self-trapped hole (STH) states. Calcuations based on hybrid and self-interaction-corrected density functional theories further validate that the polarization dependence is consistent with the relative stability of two STHs. This observation implies that the STHs form primarily at the oxygen atoms involved in the original photon absorption event, thus providing the connection between incident polarization and emission wavelength. The data imposes a lower bound on the energy separation between the self-trapped hole states of ~70-160 meV, which is supported by the calculations.

By using positron annihilation spectroscopy methods, we have experimentally demonstrated the creation of isolated zinc vacancy concentrations >10 20  cm -3 in chemical vapor transport (CVT)-grown ZnO bulk single crystals. X-ray diffraction ω-rocking curve (XRC) shows the good quality of ZnO single crystal with (110) orientation. The depth analysis of Auger electron spectroscopy indicates the atomic concentrations of Zn and O are almost stoichiometric and constant throughout the measurement. Boltzmann statistics are applied to calculate the zinc vacancy formation energies (E f ) of ~1.3-1.52 eV in the sub-surface micron region. We have also applied Fick's 2 nd law to calculate the zinc diffusion coefficient to be ~1.07 × 10 -14  cm 2 /s at 1100 °C. The zinc vacancies began annealing out at 300 °C and, by heating in the air, were completely annealed out at 700 °C.

Efficient p-type doping in CdTe has remained a critical challenge for decades, limiting the performance of CdTe-based semiconductor devices. Arsenic is a promising p-type dopant; however, reproducible doping with high concentration is difficult and carrier lifetime is low. We systematically studied defect structures in As-doped CdTe using high-purity single crystal wafers to investigate the mechanisms that limit p-type doping. Two As-doped CdTe with varying acceptor density and two undoped CdTe were grown in Cd-rich and Te-rich environments. The defect structures were investigated by thermoelectric-effect spectroscopy (TEES), and first-principles calculations were used for identifying and assigning the experimentally observed defects. Measurements revealed activation of As is very low in both As-doped samples with very short lifetimes indicating strong compensation and the presence of significant carrier trapping defects. Defect studies suggest two acceptors and one donor level were introduced by As doping with activation energies at ~88 meV, ~293 meV and ~377 meV. In particular, the peak shown at ~162 K in the TEES spectra is very prominent in both As-doped samples, indicating a signature of AX-center donors. The AX-centers are believed to be responsible for most of the compensation because of their low formation energy and very prominent peak intensity in TEES spectra.

Our investigations into the ZnO-TeO(2) system have produced a new phase, zinc(II) hexatellurium(IV) tridecaoxide, ZnTe(6)O(13), with trigonal (R3) symmetry, synthesized by repeated heating and cooling to a maximum temperature of 1053 K. The asymmetric unit consists of a Zn atom coordinated in a distorted octahedral fashion by two unique tellurium(IV) oxide units that form trigonal-bipyramidal TeO(4) and TeO(3+1) corner- and edge-shared polyhedra. Except for the Zn and an O atom, which occupy 6c positions, all atoms occupy 18f general positions.