Discovery of a new phase transition in gallium nitride at 22 GPa

Abstract

X ray data on the volume-pressure dependence V(P) for elementary cells of crystalline GaN are approximated using equations of continuum mechanics. The best approximation of function V(P) is obtained by assuming that a previously unknown continuous phase transition occurs at a pressure of 22 GPa.

Studying equilibrium values of moduli of elasticity, using models of the mechanics of finite deformations of a continuous medium

“…The bulk moduli obtained for the elasticity of copper by processing the same data set [12] using different ESes thus differ by a factor of 1.5; those obtained for aluminum, by a factor of 3 (Table 4). At the same time, such differences are not observed when calculating the bulk elasticity modulus of gallium nitride [3]. In accordance with the recommendations in [9], the description of an elastically deformed state in Euler variables is preferred below.…”

“…Nevertheless, a number of studies have attempted to extend the scope of ESMFDCM application, and its use to describe phase transitions of the first and sec ond orders in particular [1][2][3]. To describe a PT, we must divide a discrete set of experimentally deter mined values of υ(σ) into groups, each of which is associated with one phase of the crystal.…”

“…The problem then arises of growing uncertainty of the analytical description, due to reducing the number of experimentally determined values of υ(σ) relating to the same phase. The particu larly acute question of the accuracy of interpolating a low number of υ(σ) values in a complex analytic dependence arises in studies in which the existence of the PT itself is established on the basis of interpolation results and the physical characteristics of the phases after the transition, as it was done in [3,4]. Below, we answer the following questions: Why do ESes of such a distinctive type produce very close results in determin ing a bulk elasticity modulus while the elasticity mod uli of the third order predicted in different models of ESMFDCM differ greatly?…”