Infrared Spectroscopy of Protons in KTiOPO 4
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INFRARED SPECTROSCOPY OF PROTONS IN KTIOPO 4 M. K.CRAWFORD*, P. A.MORRIS*, M.ROELOFS* AND G. GASHUROV** *E. I. du Pont de Nemours &Co., Inc., CR&D Department, Experimental Station, Wilmington, DE 19880-0356 **Airtron, Litton Systems, Inc., Morris Plains, NJ ABSTRACT We report infrared (IR) measurements of protons (H+), present as OH-, in KTiOPO 4 grown by various techniques. The IR spectra are sensitive to the growth technique used. In high temperature hydrothermally grown crystals we observe IR bands which have strongly temperature dependent linewidths. The Arrhenius-type activation energies for these linewidths are small (6-15 meV). The IR bands are polarized and this information can aid in making OH- site assignments. INTRODUCTION KTiOPO 4 (KTP) is a relatively new nonlinear optical material which is finding increasing use in second harmonic generation and electro-optic applications. In this paper we discuss the results of infrared (IR) measurements of OH- bands in KTP. (We refer the reader to the article by P. A. Morris, et al., found elsewhere in this proceedings for discussion of the electrical properties and growth of KTP.) Protons (H+), present as defects in KTP at concentrations of several hundred ppm,2 can influence the electrical properties and damage resistance of this material. Since the electrical neutrality of the crystal is maintained according to the following defect equations, [K a] + [Ti3+] [H+]
=
2[O'] + [H+]
= [OH-]
(1) (2)
the H+can charge compensate K+vacancies, Kvac + , and Ti3+. The presence of Kvac + has been implicated as the underlying reason for the high ionic conductivity of KTP 3 along the c direction. In Figure 1a we show the structure of KTP with the perspective of looking "down" the c axis. The most striking aspect of the structure is the large square channels which lie parallel to the c axis. These channels are ringed by K+ions which wind helically through them. It is this structural feature which leads to the high ionic conductivity along c. Figure lb shows a different perspective of the structure, looking down the x axis. Note the -Ti-O-Ti-o- chains which lie along the c axis. Damage in KTP involves introduction of Ti3+ ions in these one dimensional chains, leading to a broad optical absorption polarized along c. 4 Thus the c axis channels, the K+ helices and the Ti-O chains are all intimately involved in determining the physical properties of KTP. Previous IR studies of protons (OH-) in KTP have observed multiple OH" bands which were attributed to different sites in the crystal. 5 Our work extends these previous studies by studying the effect of growth method on the OH- spectra, Mat. Res. Soc. Symp. Proc. Vol. 172. ©1990 Materials Research Society
342
performing fully polarized measurements and observing the detailed temperature dependence of the OH- bands.
Figure 1. Structure of KTP looking down the c axis (left); looking down the x axis (right).
EXPERIMENTAL All IR spectra were infrared spectrometer. hydrothermally or by a flux the IR measurements were cryostat.
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