Anomalous Compressibility and Amorphization in AlPO4-17, the Oxide with the Highest Negative Thermal Expansion

AlPO4-17, known as the oxide with the highest negative thermal expansion (NTE), was studied under high pressure by angle-dispersive X-ray diffraction (XRD), mid- and far-infrared (IR) spectroscopy. Upon increasing pressure, the closure of the (P-O-Al) angle destabilizes the porous AlPO4-17 structure, which drives the amorphization process. On the basis of the decrease in intensity of the XRD lines and broadening of the IR modes, the material was found to begin to amorphize near 1 GPa. XRD, mid- and far-IR analysis evidenced pressure-induced framework softening and complete irreversible amorphization near 2.5 GPa corresponding to the collapse of the pores. The bulk modulus and its first pressure derivative (B-0 = 31.2(5) GPa and B'(0) = -10.1(3)) at ambient temperature were determined by fitting a third order Birch-Murnaghan equation of state (EOS) to the pressure-volume data. The material is extremely compressible and exhibits an elastic instability. Anomalous (negative) values of B'(0) are very rare and have been observed previously for cyanides and metal-organic frameworks. Such an instability appears to be characteristic of materials, which exhibit strong NTE behavior and indicates a link between NTE and anomalous compressibility behavior. Mid-IR, far-IR, nuclear magnetic resonance, and pair distribution function analysis of the new amorphous form allow an amorphization mechanism to be proposed corresponding to a collapse of the structure around its pores retaining the columns built up of cancrinite cages and hexagonal prisms, based on alternating AlO4 and PO4 tetrahedra. An increase in coordination number of 10% of the Al atoms was observed. The pressure-induced amorphization in the strong NTE material AlPO4-17 opens the door to the development of new technological applications as crystal-amorphous nanocomposites with zero or specifically selected thermal expansion coefficients.