Quartz-type Phases Research Articles

Quantum mechanical calculations of the vibrational spectra of quartz- and rutile-type GeO2

Heat-treatment and stepwise cooling of as-delivered, water-containing quartz-type GeO2 powder resulted in transformation into a water-free form. A rutile-type modification could be prepared by impregnation of the quartz-type phase with RbOH solutions, drying and annealing. Raman- and FTIR-absorption spectra of quartz- and rutile-type GeO2 were measured and compared to quantum-mechanical ab initio calculations based on a hybrid functional using the Perdew–Burke–Ernzerhof correlation functional with 16.7% Hartree–Fock exchange density functional. Maximum and mean deviations between measured spectral bands and assigned vibrational modes are 14 and ±8 cm−1 for the quartz-type and 30 and ±13 cm−1 for the rutile-type polymorphic form. Water is incorporated into GeO4 entities of quartz-type GeO2; a water-free and structurally stable form can be prepared by a heating up to 1,425 K, tempering at 1,323 K and stepwise cooling. Spectral bands not explained by the calculations suggest defects and distortions in both quartz- and rutile-type structures, in case of the quartz-type one by incomplete transformation into an ideal structure after removing the water, whereas the rutile-type modification most probably incorporates Rb during its synthesis.

High-temperature, structural disorder, phase transitions, and piezoelectric properties ofGaPO4

Gallium orthophosphate was studied at high temperature up to $1303\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by total neutron scattering and $1173\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by piezoelectric measurements. Rietveld refinements at $1223\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ confirm the stability of the structural distortion in the $\ensuremath{\alpha}$-quartz-type phase with an average tilt angle $\ensuremath{\delta}=18.8\ifmmode^\circ\else\textdegree\fi{}$ at this temperature. In contrast, reverse Monte Carlo (RMC) refinements of total neutron scattering data indicate that, whereas the degree of structural disorder initially slowly varies over a very large temperature interval in the $\ensuremath{\alpha}$-quartz-type phase, an increase in disorder is observed beginning above $1023\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Piezoelectric measurements indicate that the quality factor $(Q)$ of $\mathrm{Ga}\mathrm{P}{\mathrm{O}}_{4}$ resonators remains stable up to this temperature above which the piezoelectric properties of the material degrade. This degradation can be correlated to the increase in structural disorder. RMC refinements indicate that the high-temperature $\ensuremath{\beta}$-cristobalite-type phase at $1303\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is characterized by significant thermally induced disorder with oxygen atom density forming a continuous ring around the vector joining neighboring gallium and phosphorous atoms. Gallium phosphate may be expected to retain its piezoelectric properties up to within $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ of the phase transition temperature and as a consequence be used in applications at temperatures slightly above $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

High-pressure X-ray investigation of the moganite- and quartz-type phases of phosphorus oxynitride

The high-pressure behavior of the moganite-type polymorph of phosphorus oxynitride PON (monoclinic, I2/ a, Z=12) has been determined by angle dispersive X-ray powder diffraction in a diamond anvil cell to 50 GPa. The pressure variations of the three cell parameters display a distinct slope change: the axial compressibilities are about three times larger below 9 GPa than above this pressure. This behavior is completely reversible. The molar volume of the moganite-type phase under pressure remained larger than that of the quartz-type phase determined under similar conditions. The pressure-induced amorphization of the quartz- and moganite-type phases is compared: while it was reversible upon decompression for moganite it was not for the quartz-phase. This difference in behavior is attributed to local configurations in the structures of the two phases.

A moganite-type phase in the silica analog phosphorus oxynitride

A new polymorph of phosphorus oxynitride (PON) a silica analog has been recovered at ambient pressure by quenching after a treatment at 850 degrees C under a pressure of 2.5 GPa using cristobalite- or quartz-type phases as starting materials. This PON polymorph is a thermodynamically stable phase with its own stability field in a P-T diagram. The structure of this PON phase was refined by the Rietveld method from an X-ray powder diffractogram. It is isostructural with moganite (SiO 2 ). The discovery of this PON polymorph should stimulate a renewed interest in the occurrence of this phase in the silica system, because moganite may have a small but defined P-T stability field. These results confirm the structure of moganite as a new structure-type in AX 2 compounds.

Crystal structure and high pressure behaviour of the quartz-type phase of phosphorus oxynitride PON

Phosphorus oxynitride PON is isoelectronic with SiO 2 and both exhibit cristobalite- and quartz-type phases. The α-quartz phase of PON was prepared by submitting the β-cristobalite phase to 4.5 GPa and 750°C in a belt type apparatus. The structure at ambient was refined using time-of-flight neutron powder diffraction data on a quenched sample. The structure (trigonal, P3 221, a=4.757(4), c=5.2460(2) Å, Z=3) is essentially the same as that of α-quartz SiO 2. The atomic positions and temperature factors of phosphorus, oxygen and nitrogen were obtained. Oxygen and nitrogen are completely disordered over the 6(c) sites. The behaviour of α-quartz-type PON under pressure was determined by angle dispersive X-ray powder diffraction in a diamond anvil cell up to 48 GPa. The cell parameters were measured as a function of pressure. A transition to an α-quartz II phase occurs at close to 20 GPa; the main change is a small, but distinct increase in the c/ a ratio. Progressive and irreversible amorphisation of PON was observed above 30 GPa, and no diffraction lines could be detected above 42 GPa. For both PON and SiO 2, the α-quartz-type phases undergo phase changes under pressure to quartz II phases followed by amorphisation. This pressure study reinforces the parallel behaviour of the two compounds.

Erratum: Crystal structures of the low-temperature quartz-type phases of SiO2 and GeO2 at elevated pressure

Article Erratum: Crystal structures of the low-temperature quartz-type phases of SiO2 and GeO2 at elevated pressure was published on December 1, 1992 in the journal Zeitschrift für Kristallographie - Crystalline Materials (volume 202, issue 1-4).

Crystal structures of the low-temperature quartz-type phases of SiO2 and GeO2 at elevated pressure

Crystal structures of the low-temperature quartz-type phases of SiO₂ and GeO₂ at elevated pressure

Crystal structures of the low-temperature quartz-type phases of SiO2 and GeO2 at elevated pressure

Crystal structures of the low-temperature quartz-type phases of SiO₂ and GeO₂ at elevated pressure

Crystal structures of the low-temperature quartz-type phases of SiO2and GeO2at elevated pressure

Lattice parameters and crystal structures of the low temperature quartz-type (low-quartz) forms (space group P3 1 21) of SiO 2 and GeO 2 were refined from single crystal X-ray diffraction data under hydrostatic pressures up to 10.2GPa for SiO 2 and 5.57GPa for GeO 2 . R W (F) values range from 2 to 5%. GeO 2 -quartz is an almost perfect high-pressure model for SiO 2 -quartz:At 10GPa the geometry of the SiO 2 -quartz structure approaches that of GeO 2 at ambient pressure.