The high-pressure behavior of α-quartz, oxynitride, and nitride structures

Abstract

The high-pressure behavior of a-quartz SiOz and GeO z , oxynitrides SizNzO and GezNzO and aandp-Si 3 N 4 depends markedly on the nature of the linking of the coordination polyhedra in their crystal structures. Where bond angles can vary between neighboring polyhedra linked at one corner, relative tilting or rotations can occur modifying the aspect of the three-dimensional framework. Such fine structural rearrangements can dominate the reponse of the crystal structure to applied stresses as induced by the application of high pressures or by temperature changes. Direct evidence for this is available from structural measurements at high pressure using time-offlight neutron diffraction with multicomponent profile refinement procedures, where atomic position changes have been determined up to pressures of -3 X 10 9 Pa. The crystal structures studied are the flexible a-quartz structures of SiOz and GeOz, the partially flexible structures SizN zO and GezN zO, and the rigid structures a-and P-Si 3 N 4 • The volume compressibility coefficients K" are greatest for a-quartz SiOz (274 X 10 -13Pa -I) and GeO z (256 X 10 -13 Pa -I) with the Si-O-Si and Ge-O-Ge angles varying the most; Kv is intermediate for SizNzO (79 X 10 "Pa I) and GezNzO (99 X 10 -13 Pa -I) where the Si-O-Si and Ge-O-Ge angles vary appreciably while the bond angles to the nitrogens change only slightly; K" is least for a-Si3N4 (35.5 X 10 -13 Pa -I) andp-Si 3 N 4 (27.5 X 10-13 Pa -I) where no bond angles vary appreciably. For all of the structures examined, bond length changes with pressure are very small in comparison to bond angle changes, which can be an order of magnitude greater. Each structural type behaves in a characteristic way. For example, SizNzO and GezNzO exhibit almost identical responses to pressure.The experimental data have been taken using time-offlight neutron diffraction with powder samples in a hydrostatic, supported piston-cylinder ce11. 7 • H The details of the individual studies of a-andp-Si 3 N 4 (Ref. 9-10), Si2N 20 (Ref. 11), Ge1NzO (Ref. 12), and the a-quartz forms ofSi01 and GeOz (Ref. 7) are published elsewhere.

In situstudy of amorphization and changes in coordination inSi2N2Oat high pressure

“…35 The observed changes in intensity arise from the structural changes that occur over this pressure range and, in particular, to the important reduction in the Si-O-Si angle. 26,27 Very important changes in the infrared spectrum occur above 25 GPa. Certain modes strongly decrease in intensity and disappear with further increases in pressure.…”

“…Si 2 N 2 O was previously studied up to 2.3 GPa by neutron diffraction. 26,27 The main compression mechanism was found to be a cooperative rotation of the SiN 3 O tetrahedra with a consequent reduction in the Si-O-Si angle. Shock-recovery experiments indicate that Si 2 N 2 O is stable up to about 28 GPa and becomes gradually amorphous between 34 and 41 GPa.…”

Extending the methodology of X-ray crystallography to allow imaging of micrometre-sized non-crystalline specimens

“…13) and ␤-Si 3 N 4 (250 GPa; ref. 24). Our calculations of the bulk moduli of the ␣-and ␤-phase gave 227 GPa and 249 GPa, respectively.…”

Analytic Equation of State and Thermodynamic Properties for α-, β-, and γ-Si3N4Based on Analytic Mean Field Approach

“…(4) by fitting the compression experimental data of α-, β-, and γ-Si 3 N 4 [8,13,14]. The determined values of these parameters are listed in Table. Furthermore, we calculate the numerical results of the pressure dependence of the molar volume at room temperature by using the AMFP, and compare our results with the available experimental data [8,13,14] in Figs. 1-3.…”

“…It has a hardness comparable to the hardest known oxide (stishovite, a high-pressure phase of SiO 2 ) [2][3][4][5][6] and significantly greater than the hardness of α-and β-Si 3 N 4 [7]. In recent years, many experiments on the properties of Si 3 N 4 have been performed, and their theories have been studied [8][9][10][11][12][13][14][15][16][17][18]. Among all the properties of Si 3 N 4 studied so far, the thermodynamic properties have been an intriguing subject.…”