The new double perovskite oxides Sr2Mn1−xNixTeO6 with x = 0.25, 0.5 and 0.75 have been synthesized in polycrystalline form by a conventional solid-state reactions process at 1180 °C, in the air. The structural and vibrational properties of these materials were studied by means XRPD, Raman, and IR spectroscopy. It has been proven that all the materials show typical double perovskite structures with a monoclinic symmetry, but with two different space group, P21/n for the compositions (x = 0.25 and 0.5), while the composition (x = 0.75) crystallizes in the space group I2/m. The lattice parameters obtained are in agreement with those of the two pure extremes Sr2MnTeO6 and Sr2NiTeO6. The monoclinic structural distortion involves long range ordering between Te6+ (in 2b site) and a random mixture (Mn2+/Ni2+) (in 2a site) for the two compositions (x = 0.25 and 0.5) that crystallize in P21/n. For the material (x = 0.75) with I2/m, similar distortion, ordering and mixing occur at the B and B’ double perovskite sites. It was observed that Vegard Law is satisfied, taking into account the cell parameters of both extremes. The effect of the partial substitution of Mn by Ni was also seen in Raman and IR data where a significant mode shift was observed with nickel content increase.

Synchrotron X-ray diffraction was used to monitor the hydrothermal precipitation of akaganeite (β-FeOOH) and its transformation to hematite (Fe2O3) in situ. Akaganeite was the first phase to form and hematite was the final phase in our experiments with temperatures between 150 and 200 °C. Akaganeite was the only phase that formed at 100 °C. Rietveld analyses revealed that the akaganeite unit-cell volume contracted until the onset of dissolution, and subsequently expanded. This reversal at the onset of dissolution was associated with a substantial and rapid increase in occupancy of the Cl site, perhaps by OH− or Fe3+. Rietveld analyses supported the incipient formation of an OH-rich, Fe-deficient hematite phase in experiments between 150 and 200 °C. The inferred H concentrations of the first crystals were consistent with “hydrohematite.” With continued crystal growth, the Fe occupancies increased. Contraction in both a- and c-axes signaled the loss of hydroxyl groups and formation of a nearly stoichiometric hematite.

Non-stoichiometric, carbon-containing crandallite from Guatemala and plumbogummite from Cumbria have been characterized using electron microprobe (EMPA) and wet-chemical analyses, Rietveld analysis of powder X-ray diffraction (PXRD) patterns, and infrared (IR), Raman and cathodoluminescence (CL) spectroscopies. The samples contain 11.0 and 4.8 wt.% CO2, respectively. The IR spectra for both samples show a doublet in the range 1410–1470 cm–1, corresponding to CO3 vibrations. Direct confirmation of CO3 replacing PO4 was obtained from difference Fourier maps in the Rietveld analysis. Carbonate accounts for 67% of the C in the plumbogummite and 20% of the C in the Guatemalan crandallite, the remainder being present as nano-scale organic carbon. The CO3 substitution for PO4 is manifested in a large contraction of the tetrahedral volume (14–19%) and by a contraction of the a axis, analogous to observations for carbonate-containing fluorapatites. Stoichiometric crandallite from Utah was characterized using the same methods, for comparison with the non-stoichiometric, carbon-bearing phases.

The crystal structure of an isotropic, single phase, Ti-rich schorlomite garnet, ideally Ca3Ti4+ 2(Fe3+ 2Si)O12, from Magnet Cove, Arkansas was refined with the Rietveld method, space group $Ia\overline 3 d$ , and monochromatic synchrotron high-resolution powder X-ray diffraction data. Electron-microprobe analysis gave an average composition {Ca2.92Na0.04Mn2+ 0.03Mg0.01}Σ3[Ti1.04Fe3+ 0.46Mg0.18Fe2+ 0.16Zr4+ 0.13V3+ 0.04]Σ2(Si2.21Fe3+ 0.71Al0.07)Σ3O12, and corresponds to the general garnet formula of [8] X 3 [6] Y 2 [4] Z 3 [4]O12. Schorlomite is the dominant component, but the composition contains significant amounts (>15 mol.%) of andradite, Ca3(Fe3+ 2)Si3O12, morimotoite, Ca3(Ti4+Fe2+)Si3O12, and morimotoite-(Mg), Ca3(Ti4+Mg)Si3O12. The crystal structure model was refined well using isotropic and anisotropic displacement parameters. Using isotropic displacement parameters, the χ 2 and R (F 2) Rietveld refinement values are 1.148 and 0.0742, respectively. The unit-cell parameter, a = 12.18599(1) (Å), is large for a natural schorlomite for which complete structural data are available. The bond distances are average <Ca–O> = 2.4461, Ti–O = 2.0085(5), Si–O = 1.7022(5) Å, and site occupancy factors (sofs) for Ca(sof) = 0.963(1), Ti(sof) = 1.045(1), and Si(sof) = 1.150(2). Comparison of schorlomite data from the type locality in Magnet Cove with morimotoite from Ice River, Canada and the type locality in Japan show that they are quite similar and cast doubts as to morimotoite being different from schorlomite.

The crystal structure of a morimotoite garnet, ideally Ca3(Ti4+Fe2+)Si3O12, from the Ice River alkaline complex, British Columbia, Canada was refined by the Rietveld method, space group $Ia\overline 3 d$ , and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Electron-microprobe analysis indicates a homogeneous sample with a formula {Ca2.91Mg0.05Mn2+ 0.03}Σ3[Ti1.09Fe3+ 0.46Fe2+ 0.37Mg0.08]Σ2(Si2.36Fe3+ 0.51Al0.14)Σ3O12. The HRPXRD data show a two-phase intergrowth. The reduced χ 2 and overall R(F 2) Rietveld refinement values are 1.572 and 0.0544, respectively. The weight percentage, unit-cell parameter (Å), distances (Å), and site occupancy factors (sofs) for phase-1 are as follows: 76.5(1)%, a = 12.156 98(1) Å, average <Ca–O> = 2.4383, Ti–O = 2.011(1), Si–O = 1.693(1) Å, Ca(sof) = 0.943(2), Ti(sof) = 0.966(2), and Si(sof) = 1.095(3). The corresponding values for phase-2 are 23.5(1)%, a = 12.160 67(2) Å, average <Ca–O> = 2.452, Ti–O = 1.988(3), Si–O = 1.704(3) Å, Ca(sof) = 1.063(7), Ti(sof) = 1.187(7), and Si(sof) = 1.220(8). The two phases cause strain that arises from structural mismatch and gives rise to low optical anisotropy. Because the two phases are structurally quite similar, a refinement using a single-phase model with anisotropic displacement parameters shows no unusual displacement ellipsoid for the O atom that requires a “split O-atom position”, as was done in previous studies.

The crystal structure of one isotropic [(1) Brazil] and three birefringent spessartine samples [(2) California, (3) Tanzania, and (4) Colorado] were refined using the Rietveld method, cubic space group $Ia\overline 3 d$ , and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. The results of electron-microprobe analysis (EMPA) indicate homogeneous compositions, in terms of end-members, as follows: (1) Sps54Alm43, (2) Sps90Alm8, (3) Sps64Prp27Grs3, and (4) Sps73Alm19. Their crystal structures were modeled well as indicated by the Rietveld refinement statistics where the reduced χ 2 and overall R (F 2) values for each sample are: (1) 1.395 and 0.0329, (2) 1.082 and 0.0354, (3) 1.025 and 0.0347, and (4) 1.016 and 0.0413. Two cubic phases occur in samples 2–4, and a single cubic phase occurs in sample-1. The dominant cubic phase-1 with locality, weight fraction (%), unit-cell parameter (Å), distances (Å), and site occupancy factors (sofs) are as follows: (1) Brazil: 100%, a = 11.581 54 (1), average <Mn–O> = 2.3156, Al–O = 1.8949 (3), Si–O = 1.6376 (3) Å, Mn(sof) = 0.961(1), Al(sof) = 0.945(1), and Si(sof) = 0.936(1); (2) California: 96.67(7)%, a = 11.613 32(1), average <Mn–O> = 2.3249, Al–O = 1.8956 (4), Si–O = 1.6416 (4) Å, Mn(sof) = 0.951(1), Al(sof) = 0.946(1), and Si(sof) = 0.927(1); (3) Tanzania: 69.46(6)%, a = 11.598 45(1), average <Mn–O> = 2.3199, Al–O = 1.8964 (5), Si–O = 1.6398 (5) Å, Mn(sof) = 0.808(1), Al(sof) = 0.942(1), and Si(sof) = 0.922(1); and (4) Colorado: 98.58(6)%, a = 11.606 89(1), average <Mn–O> = 2.3204, Al-O = 1.8948 (6), Si–O = 1.6450 (6) Å, Mn(sof) = 0.949(1), Al(sof) = 0.967(2), and Si(sof) = 0.913(2). The two-phase intergrowth causes strain that arises from mismatch of the structural parameters and gives rise to strain-induced birefringence.

The cause of birefringence in several garnet-group minerals with general chemical formula, [8]X3 [6]Y2 [4]Z3 [4]O12, which was observed over 100 years ago, is unknown, although many different reasons were proposed, including symmetry lower than cubic. In this study, electron microprobe analyses (EMPA) were obtained for a Ti-rich andradite, ideally Ca3(Fe2 3+)Si3O12, from Magnet Cove, Arkansas, USA, and the results show that the sample is inhomogeneous with two distinct compositions. The crystal structure was refined by the Rietveld method, cubic space group $Ia\overline 3 d$ , and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data, which shows a mixture of three distinct cubic phases that are intergrown together and cause birefringence because of strain arising from small structural mismatch. This mixture of three cubic phases was not observed by any other experimental technique. These results have many implications, including garnet phase transitions from cubic to lower symmetry in the mantle, which has important geophysical consequences.

This paper reports the results of crystallography and crystal chemistry investigation of the (Ba1−xSrx)Y2CuO5 (“green phase”) solid solution series by X-ray powder diffraction (XPD) and neutron powder diffraction techniques. The single phase regions for (Ba1−xSrx)Y2CuO5 were determined to be 0⩽x⩽0.3 for samples prepared at 810 °C in 100 Pa pO2, and 0⩽x⩽0.7 for samples prepared at 930 °C in air. All single phase (Ba1−xSrx)Y2CuO5 samples are isostructural to BaY2CuO5 and can be indexed using an orthorhombic cell with the space group Pnma. Lattice parameters, a,b,c and the cell volume, V, of the (Ba1−xSrx)Y2CuO5 members decrease linearly with increasing Sr substitution (x) on the Ba site. The general structure of (Ba1−xSrx)Y2CuO5 can be considered as having a three-dimensional interconnected network of [YO7],[(Ba,Sr)O11], and [CuO5] polyhedra. The copper ions are located inside distorted [CuO5] “square” pyramids. These pyramids are connected by the [Y2O11] groups that are formed from two monocapped [YO7] trigonal prisms sharing a triangular face. The Ba2+ ions are found to reside in distorted 11-fold coordinated cages. The oxygen sites are essentially fully occupied. XPD reference patterns of two members of the series, (Ba0.3Sr0.7)Y2CuO5 and (Ba0.7Sr0.3)Y2CuO5, were prepared for inclusion in the powder diffraction file.