X-ray diffraction: Powder X-ray diffraction (PXRD) patterns were recorded from 10 to 80° (2θ) with a step size of 0.02° and a scan time of 2s per step using a D8 Advance Bruker diffractometer (Cu Kα1,2 radiation, LynxEye detector) at room temperature in Bragg-Brentano geometry. Finely-ground powder samples were dispersed on silicon wafers from an ethanol mull. In situ variable temperature diffraction (VT-XRD) data were collected using a Bruker D8 Advance diffractometer (CuKα1,2 radiation, Vantec detector) equipped with an HTK1600N Anton Paar furnace. Powder samples were deposited on a platinum ribbon from an ethanol mull, and measurements were collected in Bragg-Brentano geometry with the sample under vacuum, from room temperature up to 1600 o C in the 2θ range 10-120 o with a step size of 0.02° and a scan time of 2s per step. High-resolution synchrotron powder diffraction (SPD) data were recorded on 11-BM diffractometer at Argonne National Laboratory, U.S.A. in the range of 2θ from 1 o to 50 o with a 0.0001 o step size using a wavelength of 0.4578 Å at room temperature. The measured powders were obtained by crushing one bead for each composition and loading into glass capillaries with a diameter of 0.3 -0.5mm. Rietveld refinements were performed using TOPAS Academic version 6. Thermal analysis: Differential scanning calorimetry (DSC) data were acquired on heating using a Perkin Elmer Setaram MULTI HTC 1600 Thermal Analyzer with a heating rate of 10 °C.min -1 under a constant argon flow (20 cm 3 min -1 ). Experiments were conducted on glass powders contained in a platinum vessel, except for x=0.6 which was conducted on a bulk (single bead) sample.