Discovery of a Sodium-Ordered Form of Na₃V₂(PO₄)₃ below Ambient Temperature

Abstract

The crystal structure of the NASICON Na3V2(PO4)3 phase (NVP) has been investigated as a function of temperature. Combining laboratory and synchrotron X-ray powder diffraction with single crystal X-ray diffraction, we demonstrate the presence of four polymorphs of NVP from 30 to 225 °C. While the high temperature γ-NVP crystallizes in the classical rhombohedral cell (S.G. R3̅c, a = 8.73382(4) Å, c = 21.91438(17) Å), the low temperature α-NVP undergoes a monoclinic distortion (S.G. C2/c, a = 15.1244(6) Å, b = 8.7287(3) Å, c = 21.6143(8) Å, β = 90.163(2)°) together with an ordering of the Na atoms. In the middle temperature range, two incommensurate modulated structures (β- and β′-NVP) are also reported for the first time.

Improving the energy density of Na3V2(PO4)3-based positive electrodes through V/Al substitution

“…1 Electrochemical signatures of Na 3 V 2 (PO 4 ) 3 electrodes upon Na + extraction (left) or Na + insertion (right). 15,31 Order-disorder transitions within the Na + sub-lattice are typical of NAtrium Super Ionic CONductors (NASICON) and are associated with a randomization of the Na + positions and a lowering of the activation energy for ionic conduction. 15,17 This leads to a set of superstructure diffraction peaks, indicated in Fig.…”

“…Minor "impurities" refer to traces of V 2 O 3 or to the incommensurate form of NVP (see31 ). The "new" diffraction peaks ($) refer to d hkl values not indexable in the rhombohedral R 3c space group.…”

Arrhenius Behavior of the Bulk Na-Ion Conductivity in Na3Sc2(PO4)3 Single Crystals Observed by Microcontact Impedance Spectroscopy

“…13 Also with the enlarged monoclinic cell, recently suggested by Chotard et al for NVP, it is not possible to index all observed Bragg peaks. 3 The comparison of their reconstructed reciprocal space layer 0 kl with those of this study reveals substantial differences with the superstructure peaks appearing along the c * direction; for NVP, they appeared along the b * direction. The only possibility to index all Bragg peaks for NSP fit with a 2 a , 2 b , c trigonal cell.…”

“…As an example, in the case of Na 3 V 2 (PO 4 ) 3 (NVP) a phase transition sequence from an ordered monoclinic α form (space group (SG) C2/ c , a = 15.1244(6) Å, b = 8.7287(3) Å, c = 21.6143(8) Å, β = 90.163(2)) to the classical disordered rhombohedral cell ( SG R 3̅ c , a = 8.73382(4) Å, c = 21.91438(17) Å) with two intermediate incommensurate modulated structures β and β′ was described. 3 A further complication arises due to the different and conflicting behavior of powders and single crystals of NASICON-materials. 4 For polycrystalline samples of Na 3 Sc 2 (PO 4 ) 3 (NSP), fabricated at high temperatures, two phase transitions are reported 5−7 with the room temperature (RT) structure often being described using a monoclinic cell with space group symmetry C 2/ c .…”

Four-Dimensional Incommensurately Modulated Structure of Thulium Polyphosphate

“…In recent years, many commensurately or incommensurately modulated compounds with intriguing structure and physical properties have been reported. [8][9][10] Rare-earth phosphates with the general formula Ln(PO 3 ) 3 (Ln=Sc, Y, La− Lu) are one of the most attractive family mainly for their optical applications. [11][12][13] Their good chemical and thermal stability as well as relatively simple preparation ensure their wide application in many fields.…”