Vitrification of LiCa(PO3)3 by Mechanical Method

A new glass state of LiCa(PO3)3 was obtained from its crystalline phase via the planetary ball milling (PBM) technique, a high-energy mechanical process carried out in the solid state at room temperature. The product of PBM was analyzed as a function of the milling time using x-ray diffraction, 31P and 7Li nuclear magnetic resonance (NMR), and differential scanning calorimetry (DSC). The amorphized material shows DSC signatures of glass transition and crystallization. This glass is produced in an excited structural state, relaxing an excess of enthalpy below Tg, akin to hyperquenched glasses. This relaxation seems independent of the primary relaxation activated during the glass transition. The collapse of the crystal structure as a function of the milling time was monitored by 31P-NMR, revealing the disappearance of resonances from P-crystallographic sites and the appearance of Qn resonances in disordered domains. The crystal-to-amorphous transition seems to proceed as a continuous process developed by accumulation of defects and bond distortions during the early stages of milling (< 5 h). During this period, 31P-NMR shows that the amorphized material is not produced in its final structural state, but there is a progressive increase in the degree of local disorder around Q2 groups. Also, DSC shows a correlated increment in the excess of enthalpy without a well defined glass transition, despite that a significant fraction of the milled material (80%) has already transitioned to a distorted state. Above 5 h of milling, structural and thermal parameters attain a stable regime and a glass transition is clearly observed (Tg = 383 ºC), indicating that the structure of the amorphous product reached a final configuration. No trace of the original crystalline material is detected after 32 h of milling. The glass produced by PBM is less stable against devitrification than their melt-quenched counterpart, showing lower Tg, Tc, Tc -Tg, and crystallization enthalpy, and has a higher Qn-disproportionation, suggesting differences in middle-range order. Static 7Li-NMR demonstrates that Li+ mobility is similar in glasses produced by melt-quench and PBM.