The highhyphen;pressure behavior of agr;hyphen;quartz, oxynitride, and nitride structures

摘要

The highhyphen;pressure behavior of agr;hyphen;quartz SiO2and GeO2, oxynitrides Si2N2O and Ge2N2O and agr;hyphen; and bgr;hyphen;Si3N4depends 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 threehyphen;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 timehyphen;ofhyphen;flight neutron diffraction with multicomponent profile refinement procedures, where atomic position changes have been determined up to pressures of sim;3times;109Pa. The crystal structures studied are the flexible agr;hyphen;quartz structures of SiO2and GeO2, the partially flexible structures Si2N2O and Ge2N2O, and the rigid structures agr;hyphen; and bgr;hyphen;Si3N4. The volume compressibility coefficientsKvare greatest for agr;hyphen;quartz SiO2(274thinsp;times;thinsp;10minus;13Paminus;1) and GeO2(256times;10minus;13Paminus;1) with the Sihyphen;Ohyphen;Si and Gehyphen;Ohyphen;Ge angles varying the most;Kvis intermediate for Si2N2O (79thinsp;times;thinsp;10minus;13thinsp;Paminus;1) and Ge2N2O (99times;10minus;13Paminus;1) where the Sihyphen;Ohyphen;Si and Gehyphen;Ohyphen;Ge angles vary appreciably while the bond angles to the nitrogens change only slightly;Kvis least for agr;hyphen;Si3N4(35.5thinsp;times;thinsp;10minus;13thinsp;Paminus;1) and bgr;hyphen;Si3N4(27.5times;10minus;13Paminus;1) 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, Si2N2O and Ge2N2O exhibit almost identical responses to pressure.