On the pressure-induced loss of crystallinity in orthophosphates of zinc and calcium

Abstract

A recently suggested mechanism for the stress memory of various metal phosphates is investigated experimentally. Based on first-principles simulations [N. J. Mosey et al., Science 307, 1612 (2005)], it had been argued that atoms with flexible coordination, such as zinc or heavy-metal cations, act as network-forming agents, undergoing irreversible pressure-induced changes in bonding that lead to increased connectivity between phosphate anions. In the present study, orthophosphates of zinc and calcium were exposed to high pressures on surfaces and in diamond anvil cells. An additional set of first-principles simulations was accomplished on alpha-orthophosphate of zinc, which suggested that this material was already cross-linked before compression but that it nevertheless underwent a reversible coordination change under pressure in agreement with the experimental results presented here. Raman spectra indicate an irreversible, pressure-induced loss of long-range crystallinity. The pressures required to induce these changes are around 7 GPa for the zinc phosphates, while they are close to 21 GPa for the calcium phosphates. Hydrogenation of the metal phosphate lowers the threshold pressure by approximately 2-3 GPa in both cases. Moreover, alpha-orthophosphate of zinc could be partially amorphisized under nonisotropic pressure on copper foils.

Effects of Temperature and Pressure on ZDDP

“…Present results clearly show that decomposition is dominant at high pressure. However, the simple zinc phosphate may cross link at high pressures as suggested in a recent experiment [14]. The thermal decomposition of ZDDP upon heating under pressure is in agreement with the results obtained from thermal treatment of ZDDP at ambient pressure.…”

“…Under the current experimental P-T conditions, there is no substantial spectroscopic evidence to confirm the formation of a network glass structure as the result of cross-linkage via the phosphate groups. The results reported here show that the cross-linkage of phosphate groups in decomposed ZDDP may not be induced by pressure alone as suggested from the computer simulation [6], even though it was indeed observed in the model zinc phosphate system [14]. Therefore, the proposed molecular Fig.…”

“…It is noteworthy that the theoretical calculations employed a simple model system, the zinc phosphate. Experimental high pressure study [14] of this model system at room temperature indeed found evidences of pressure-induced cross-linking at the predicted pressure range. Therefore, the theoretical prediction was successful for the model system but did not fully account for the complexity of ZDDP.…”

Smart materials behavior in phosphates: Role of hydroxyl groups and relevance to antiwear films

“…This is to be expected as isolated PO 4 groups have already been shown to be stable in the pressure range investigated here. 2,22 Verification of the transition pressures suggested by the IR transmission spectroscopy can be done with the help of the XRD analysis.…”

“…It was also argued that the reversible coordination change at the cation site could explain the extraordinary stiffening ͑smart behavior͒ observed in the earlier nanoindentation experiments. 13 Subsequent high pressure experiments 12,22 provided evidence for a reversible pressure-induced cation coordination increase. They also indicated that strong stiffening occurs even without coordination change at the cation site.…”