ORIGINAL PAPER Experimental Electron Density of Ammonium Dihydrogen Phosphate in the Paraelectric as well as Antiferroelectric Phases by the Maximum Entropy Method Rajul Ranjan Choudhury • R. Chitra • Fre ´de ´ric Capet • Pascal Roussel Received: 20 June 2014 / Accepted: 8 October 2014 / Published online: 19 October 2014 Ó Springer Science+Business Media New York 2014 Abstract The experimental electron density of ammo- nium dihydrogen phosphate (ADP) crystal in the para- electric phase (155 K) as well as antiferroelectric phase (100 K) is obtained from its high resolution X-ray dif- fraction data using the maximum entropy method. Marked redistribution of electron density has been observed in ADP crystals as the crystal temperature is lowered below the phase transition temperature Tc = 148 K. The nature of very strong O–H–O hydrogen bonds between phosphate anions changes from an ideal covalent interaction to a polar covalent interaction as the temperature is altered from 155 to 100 K. The influence of intermolecular interaction like the dipolar interaction on the electron density particularly in the intermolecular region is clearly visible in the elec- tron density maps. One of the most striking features of the electron density of ADP is the presence of non nuclear maxima (NNM) within the ‘‘ab’’ planes. It is argued that the appearance of these NNMs is a normal consequence of the chemical bonding between homonuclear groups in ADP. Keywords Molecular electron density  Maximum entropy method  Hydrogen bonds Introduction Ammonium dihydrogen phosphate (ADP) is a member of the very extensively studied and technologically relevant potassium dihydrogen phosphate (KDP) family of hydro- gen bonded ferroelectric crystals [1–3]. The basic frame- work of crystals belonging to this family is constructed by a network of zigzag hydrogen bonded chains of phosphate anions (H 2 PO 4 -1 ) along the ‘‘ab’’ plane (Fig. 1a). The cat- ions (potassium ions K ? in case of KDP and ammonium ion NH 4 ? in case of ADP) occupy the interspaces of the zigzagged phosphate chains and they interconnect the adjacent anionic networks along the c-axis (Fig. 1b). The O–H–O hydrogen bonds between the phosphate anions are very strong and in the high temperature paraelectric phase its H atoms are dynamically disordered between two equivalent sites about the bond center (Fig. 1). Structural phase transitions in the crystals belonging to this family result primarily due to freezing of this dynamic disorder of H atoms. KDP crystal which is the most well studied crystal of this family undergoes a ferroelectric phase transition at 123 K. Unlike KDP crystals ADP was found to undergo an antiferroelectric phase transition at 148 K (Tc) [2]. This difference in the nature of the structural phase transition has been attributed to the presence of additional N–H–O hydrogen bonds in ADP crystals (Fig. 1). Crystal structure of ADP both above and below Tc is very well known [4, 5]. Electron density which is considered to be the glue of chemistry plays a key role in determining the crystal structure. Hence electron density is expected to undergo a significant change during a structural phase transition in a crystal. A systematic study of change in electron density across the phase transition point can provide us with an important insight into the phenomenon of structural phase R. R. Choudhury (&)  R. Chitra Solid State Physics Division, Bhabha Atomic Research Center, Trombay, Mumbai 400 085, India e-mail: rajulranjan@gmail.com F. Capet  P. Roussel CNRS, UMR8181, UCCS, Unite ´ de catalyse et de chimie du solide, ENSCL-USTL, Universite ´ Lille Nord de France, BP90108, 59652 Villeneuve d’Ascq, France 123 J Chem Crystallogr (2014) 44:586–596 DOI 10.1007/s10870-014-0553-z