c/a on biaxial stress 
a. + - experiment, solid line - calculated. Calculations were performed based on equation from ref. [5]. For pure biaxial strain c/a=-2
/(1-). For combination of biaxial and hydrostatic strain a=(1-bC)(1+(1-)
/E)-1, c=(1-bC)(1-2/E)-1, where - Poisson ratio, E - Young modulus, C- impurity concentration, b - contraction coefficient. 
