Monoclinic RbD 2 PO 4 : Room temperature synthesis, chemical and structural stability upon heating Cristian E. Botez a, * , Masoud Mollaee a , Andres J. Encerrado Manriquez a , Michael P. Eastman b a Department of Physics, University of Texas at El Paso, 500 W. University Avenue, El Paso, TX 79968, USA b Department of Chemistry, University of Texas at El Paso, 500 W. University Avenue, El Paso, TX 79968, USA highlights  P2 1 /m monoclinic RbD 2 PO 4 is structurally and chemically stable up to 210  C.  Unit cell thermal expansion of monoclinic RbD 2 PO 4 is anisotropic.  Volume of monoclinic RbD 2 PO 4 is about 1% greater than that of hydrogenated RDP.  Room temperature RbD 2 PO 4 is isomorphic with intermediate-temperature RbH 2 PO 4 .  RbD 2 PO 4 decomposes chemically via dehydration at 240  C. article info Article history: Received 22 April 2013 Received in revised form 15 September 2013 Accepted 29 September 2013 Keywords: Crystallisation X-ray scattering Rietveld analysis Phase transitions abstract Monoclinic RbD 2 PO 4 polycrystals (DRDP) were synthesized via the room temperature crystallization of RbH 2 PO 4 (RDP) dissolved in D 2 O. Powder X-ray diffraction (XRD) data collected at T ¼ 25  C indicate that this deuterated compound crystallizes in spacegroup P2 1 /m with unit cell parameters a ¼ 7.688  A, b ¼ 6.192  A, c ¼ 4.781  A and b ¼ 109.02  , and is isomorphic with the intermediate-temperature phase of its hydrogenated counterpart RDP. We found no evidence of a previously reported [Phase Transitions 80, 17 (2007)] polymorphic phase transition in DRDP upon heating from room temperature to 210  C. All lattice parameters vary smoothly within this temperature range, demonstrating that the P2 1 /m phase persists upon heating. In addition, the unit cell volume of monoclinic DRDP is w1% greater than that of its RDP polymorph at all temperatures between 150  C and 210  C, which indicates the absence of sig- nificant deuterium-hydrogen isotope exchange. Further heating to 240  C leads to the thermal decom- position of the title compound via dehydration. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Phosphate-based solid acids MH 2 PO 4 (M ¼ Cs, Rb, K) have been investigated for their ferroelectric properties [1,2], as well as for their superprotonic behavior at intermediate temperatures [3,4]. In this latter context, several studies have been performed, and much progress has been made in terms of clarifying the structural and chemical modifications undergone by some of these compounds (e.g. CsH 2 PO 4 (CDP) and RbH 2 PO 4 (RDP)) upon heating [5e9]. For example, it is now widely accepted that the three-order-of- magnitude jump in proton conductivity exhibited by CDP and RDP (when heated above 237  C and 294  C, respectively) stems from a monoclinic (P2 1 /m) / cubic (Pm3m) polymorphic transi- tion. Yet, this conclusion followed a two-decade-long debate, where several groups claimed that the superprotonic behavior of phosphate solid acids was rather due to heating induced dehy- dration and chemical modifications [10,11]. The sequence of structural and chemical modifications undergone by phosphate solid acids is difficult to track, mainly because of its dependence on the sample environment (humidity, pressure, etc) [8,10]. Moreover, heating induced twinning and cracking of single RDP and CDP crystals, as well as temperature gradients [12] further complicate the identification of the intermediate temperature phases of these compounds. It is reasonable to assume that such issues carry over to deuterated solid acids, MD 2 PO 4 (M ¼ Cs, Rb, K), materials that have more recently come under scrutiny because of new techno- logically important properties imparted by the replacement of hydrogen by deuterium in some of their phases. For example, * Corresponding author. Fax: þ1 915 747 5447. E-mail address: cbotez@utep.edu (C.E. Botez). Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.09.039 Materials Chemistry and Physics 143 (2014) 605e610