Abstract—High Pressure Raman scattering measurements of KDP:Mn were performed at room temperatures. The X-ray powder diffraction patterns taken at room temperature by Rietveld refinement showed that doped samples of KDP-Mn have the same tetragonal structure of a pure KDP crystal, but with a contraction of the crystalline cell. The behavior of the Raman spectra, in particular the emergence of a new modes at 330 cm -1 , indicates that KDP:Mn undergoes a structural phase transition with onset at around 4 GP. First principle density-functional theory (DFT) calculations indicate that tetrahedral rotation with pressure is predominantly around the c crystalline direction. Theoretical results indicates that pressure induced tetrahedral rotations leads to change tetrahedral neighborhood, activating librations/bending modes observed for high pressure phase of KDP:Mn with stronger Raman activity. Keywords—Dipotassium molybdate; High pressure; Raman scattering, Phase transition; ab initio I. INTRODUCTION HE potassium dihydrogen phosphate crystal (KDP) exhibits interesting physical properties such as ferroelectricity and ferroelasticity and is well known to present a series of phase transitions [1]. At atmospheric pressure and temperature below 60 K, KH 2 PO 4 system is monoclinic Cs i with i=1,2,3 or 4; in the temperature range 60 – 122 K, this system is ferroelectric with a orthorhombic structure belonging to Fdd2 (C 2v 19 ) space group; and in the temperature range 122 – 453 K, KDP is paraelectric with a tetragonal structure belonging to I-42d (D 2d 12 )space group. Studies by X- ray diffraction and dielectric measurements have showed that KDP undergoes structural phase transitions at high pressure [2-4]. X-ray diffraction pattern show that the crystal system of phase V (5 > P >7 GPa) is orthorhombic, and the volume reduction for the IV-V phase transition (that occurs at 4 GPa) was estimated to be 2.9% at 5.4 GPa.[3] On the other hand the influence of the introduction of a dopant into KDP crystal lattice has often been studied. Firstly, because it can modify the physical properties of this material for technological applications. Secondly because certain transition metals incorporated into a crystal lattice often modify its growth habit and KDP, in particular is a model material in industrial crystallization systems. The optimization of size and shape of a sample is of fundamental importance in industrial crystallization [5-7]. Previous works have suggested that KDP doped with manganese ions (KDP:Mn) can be obtained from solutions containing manganese salts, permanganates or mixtures of both ( in proportions of 1:1, 1:2 or 2:1). W. Paraguassu is with the Departamento de Física, Universidade Federal do Pará, Belem-Pa, 60740-000, Brazil; (e-mail: wparaguassu@pq.cnpq.br). C. R. M. remédios is with the Departamento de Física, Universidade Federal do Pará, Belem-Pa, 60740-000, Brazil; (e-mail: remédios@ufpa.br). S. Guerini is with the Departamento de Física, Universidade Federal do Maranhão, São Luis-MA, 65080-040, Brazil (e-mail: sguerini@ufma.br). P.T.C. Freire is with the Departamento de Física, Universidade Federal do Ceará, P.O. Box 6030, Fortaleza-CE, 60455-900, Brazil;(e-mail: tarso@ufc.br). The effect of impurities on the growth kinetics of crystals is provided by numerous physical and chemical factors such as concentration and oxidation states of the incorporated manganese ions as well as the pH of the solutions [8]. In a previous paper [9], for Mn concentration of 4 wt.%, we have shown that each Mn ion occupy the position of the K ion substitutionally, causing two vacancies of neighboring protons. Several other works can be found in the literature reporting the impurity influence on growth kinetics, surface morphology and physical properties of KDP crystal by using techniques like: X-ray standing waves, X-ray absorption spectroscopy, Raman scattering, Electron Spin Resonance (ESR), Mossbauer, Electron Paramagnetic Resonance (EPR), Glacing angle EXAFS and others. However there is a controversial debate in the literature about the nature of the incorporation mechanism of the Mn ion in the KDP structure. Some authors have suggested that the impurity is incorporated interstitially [10-12] while other authors suggested that impurity is incorporated substitutionally in the K ion site or in the P ion site causing vacancies in order to satisfy the charge compensation [7]. In this work we report the results of High Pressure Raman scattering measurements of KDP:Mn. We will discuss the observation of a pressure induced phase transition at 4 GPa, i.e, 1 GPa lower than the one observed for pure KDP. In addition, in order to shed light on the observed modifications in the Raman spectra and in the structural changes of KDP:Mn under hydrostatic pressure, first principle density-functional theory (DFT) calculations, were performed on I-42d (D 2d 12 ) structure of KDP [9]. II. EXPERIMENTAL Single crystals of KDP doped with Mn ions were grown by slow evaporation of a supersaturated aqueous solution containing stoichiometric KH 2 PO 4 and KMnO 4 powders, at a controlled temperature of 300 K and pH = 3.8. Elemental composition analysis of resultant crystals was made by using Rutherford backscattering spectroscopy (RBS) The results of RBS analysis show that the samples prepared had Mn concentration of 0.9 % (weight). X-ray powder diffraction was carried out on a Philips X'Pert MRD diffractometer operating at 40 kV/40 mA and using Cu Kα radiation with pyrolytic graphite diffracted beam monochromator. The diffraction patterns were obtained in the angular range of 16 o – 100 o (2θ) a counting time of 15 s/step and step of 0.02 o . The sample oscillated to prevent preferred orientation effects. The diffraction patterns were analyzed by Rietveld refinement using the General structure analysis system (GSAS) [13,14]. The backscattering light was analyzed using a Jobin Yvon Triplemate 64000 micro-Raman system equipped with a nitrogen-cooled charge-coupled device (CCD) system. The excitation source was 514.5 nm radiations from an argon ion laser with a 2 cm -1 spectral resolution set by the slits. W. Paraguassu, S. Guerini, C. M. R. Remédios, P. T. C. Freire Pressure Study on Mn Doped KDP System under Hydrostatic Pressure T World Academy of Science, Engineering and Technology International Journal of Physical and Mathematical Sciences Vol:6, No:8, 2012 1037 International Scholarly and Scientific Research & Innovation 6(8) 2012 ISNI:0000000091950263 Open Science Index, Physical and Mathematical Sciences Vol:6, No:8, 2012 publications.waset.org/7834/pdf