Crystal structure, magnetic and infrared spectroscopy studies of the LiCryFe1−yP2O7 solid solution

The lithium double diphosphates LiCryFe1−yP2O7 have been investigated by X-ray diffraction, SQUID measurements and vibrational spectroscopy. The Rietveld refinements based on the XRD patterns show the existence of a continuous solid solution over the whole composition range (0⩽y⩽1.0) with a continuous evolution of the monoclinic unit cell parameters (S.G. P21). The transition metal ions connect the diphosphate anions forming a three-dimensional network with channels filled by Li+ cations expected to exhibit high mobility. All compounds order magnetically at low temperatures due the Fe–Fe interactions. The ordering temperature decreases with increasing Cr content. The slope in Curie–Weiss fits to the 1/χ vs T data in the paramagnetic domain clearly shows the existence of Fe3+ and Cr3+ in their high spin states, and a ferromagnetic component is clearly detected for y=0, 0.2 and 0.4. IR spectra have been interpreted using factor group analysis. The small shift of the frequencies is due to the influence of the chromium amount. The POP angles were estimated using the Lazarevʹs relationship.