  Origin and composition dependence of the anisotropic thermomechanical
properties are elucidated for Ba1-xSrxZn2Si2O7 (BZS) solid solutions. The
high-temperature phase of BZS shows negative thermal expansion (NTE) along one
crystallographic axis and highly anisotropic elastic properties characterized
by X-ray diffraction experiments and simulations at the density functional
theory level. Ab initio molecular dynamics simulations provide accurate
predictions of the anisotropic thermal expansion in excellent agreement with
experimental observations. The NTE considerably decreases with increasing Sr
content x. This is connected with the composition dependence of the vibrational
density of states (VDOS) and the anisotropic Gr\"uneisen parameters. The VDOS
shifts to higher frequencies between 0-5 THz due to substitution of Ba with Sr.
In the same frequency range, vibrational modes contributing most to the NTE are
found. In addition, phonon calculations using the quasi-harmonic approximation
revealed that the NTE is mainly connected with deformation of four-membered
rings formed by SiO4 and ZnO4 tetrahedra. The thermomechanical and vibrational
properties obtained in this work provide the basis for future studies
facilitating the targeted design of BZS solid solutions as zero or negative
thermal expansion material.
