Thermal expansion coefficients anisotropy of monoclinic potassium (rare-earth) double tungstates KRe(WO4)2 (Re=Gd,Y,Lu,Yb)

Potassium-(rare-earth)-double tungstates doped with rare-earth ions [R:KRe(WO₄)₂, shortly KReW, Re = Gd, Lu,Y are "passive" ions while R = Nd,Yb,Tm,Ho etc. are active trivalent rare-earth ions] are the promising materials for bulk and waveguide efficient, robust and short-pulse solid-state lasers. For the aims of heat management of such laser systems, one should know the values of linear thermal expansion coefficients a_T along the optical indicatrix axes N_p, N_m and N_g (as the real laser elements are usually oriented along these directions). Unfortunately, up to now such measurements has not been performed (in the previous papers the values of α_T obtained by X-ray analysis were reported only for crystallographic directions. The results of several dilatometric measurements is presented. The aim of present paper is the comparative experimental study of the α_T anisotropy in the isostructural KReW crystals (including measurements in the N_p, N_m and N_g directions).