| Summary: | <p>This thesis reports the synthesis and structural characteristics of solid solutions derived from KTP,KTiOPO<sub>4</sub> (space group <em>Pn2<sub>la</sub></em>, Z=8, <em>a</em>=12.814(6)Å, <em>b</em>=10.616(5)Å, <em>c</em>= 6.404(2)Å) with substitution of K by Na and Rb; Ti by Sn, Ge and Cr; and P by As. The KTiOPO<sub>4</sub> structure contains two formula units per asymmetric unit and hence two crystallographically distinct K, Ti and P sites. This makes substituent ordering possible with selective modification of the nonlinear optical properties.</p> <p>K<sub>0.5</sub>Na<sub>0.5</sub>TiOPO<sub>4</sub>, (occupancy K(1):96(7)%Na, K(2):105(8)%K) was characterised by Rietveld analysis of combined time-of-flight (TOP) neutron and X-ray powder diffraction, the X-ray diffraction data being required to distinguish between K and Na. Constant wavelength neutron powder diffraction data were used to refine the Na<sub>0.5</sub>Rb<sub>0.5</sub>TiOPO<sub>4</sub> structure (K(1):97(3)%Na, K(2):96(3)%Rb). The cations in both these materials were fully ordered over the two possible K sites, while in K<sub>0.5</sub>Rb<sub>0.5</sub>TiOPO<sub>4</sub> (K(1):71(2)%K, K(2):71(2)%Rb) the cation distribution, determined from a TOP neutron powder diffraction study, was only partially ordered. A single crystal X-ray diffraction study of an ion-exchanged K<sub>1-x</sub>Na<sub>x</sub>TiOPO<sub>4</sub> sample (x=0.58), confirmed that the cation distribution observed in the powder diffraction analysis was also present after ion-exchange at lower temperatures (350°C), (K(1):93.3(1)%Na, K(2):77.3(7)%K). An ion-exchanged crystal that had been annealed at a temperature closer to the KTiOPO<sub>4</sub> synthesis temperature gave almost identical results (K(1):94.4(6)%Na,K(2):81.0(4)%K).</p> <p>Substitution for Ti by Sn or Ge gave rise to only partial ordering. The structures of KTi<sub>0.5</sub>Sn<sub>0.5</sub>OPO<sub>4</sub>(Ti(1):63.9%Ti, Ti(2):64.6(5)%Sn) and KTi<sub>0.5</sub>Ge<sub>0.5</sub>OPO<sub>4</sub> (Ti(1):76.7(4)%Ti, Ti(2):66.1(5)%Ge) were determined through combined analysis of X-ray and TOP neutron powder diffraction data. The X-ray data were required to define the Ti(1) site as the average weighted neutron scattering length was approximately zero. An apparently random distribution of As over the two P sites was demonstrated by time-of-flight powder neutron diffraction from KTiOP<sub>0.5</sub>As<sub>0.5</sub>O<sub>4</sub>(P(1):56(1)%P, P(2):56(1)%As). Measurements on KTiOP<sub.1-x< sub="">As<sub>x</sub>O<sub>4</sub> powders indicated that the Nd:YAG second harmonic intensity increased with As. The structures of RbTi<sub>0.5</sub>Sn<sub>0.5</sub>OPO<sub>4</sub> and RbTiOP<sub>0.5</sub>As<sub>0.5</sub>O<sub>4</sub> were investigated by powder X-ray diffraction and showed similar ordering to the K isomorphs.</sub.1-x<></p> <p>Simultaneous substitution for both K and Ti was investigated by Rietveld analysis of combined X-ray and TOP neutron powder diffraction data on K<sub>0.5</sub>Na<sub>0.5</sub>Ti<sub>0.5</sub>Sn<sub>0.5</sub>OPO<sub>4</sub> (K(1):73(4.)%Na,Ti(1):66.9(3)%Ti), Na<sub>0.5</sub>Rb<sub>0.5</sub>Ti<sub>0.5</sub>Sn<sub>0.5</sub>OPO<sub>4</sub> (K(1):54(2)%Rb, Ti(1):67.0(5)%Ti), K<sub>0.5</sub>Rb<sub>0.5</sub>Ti<sub>0.5</sub>Sn<sub>0.5</sub>OPO<sub>4</sub> (K(1):77(1)%K, Ti(1):66.1(3)%Ti) and K<sub>0.5</sub>Rb<sub>0.5</sub>SnOPO<sub>4</sub>(K(1):64(2)%K).</p> <p>Cr-doped materials were studied by single crystal X-ray and TOP neutron powder diffraction. Doping a small quantity of Cr into KTP produced the KTi.<sub>95</sub>Cr.<sub>05</sub>OPO<sub>4</sub> isomorph, (3.2(6)%Cr on both Ti sites). Substitution of a larger amount of Cr produced K<sub>1.8</sub>Ti<sub>1.2</sub>Cr.<sub>8</sub>(PO<sub>4</sub>)<sub>3</sub>, (space group <em>P2<sub>l</sub>3</em>, a=9.7902(1)Å, Ti(1):69.4(6)%Ti, Ti(2):54.9(6)%Ti) isomorphous with Langbeinite. Magnetic measurements using a SQUID susceptometer found a value for the moment consistent with Cr(III).</p>
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