Structural studies on nonlinear optical materials

This thesis reports the synthesis and structural characteristics of solid solutions derived from KTP,KTiOPO4 (space group Pn2la, Z=8, a=12.814(6)Å, b=10.616(5)Å, c= 6.404(2)Å) with substitution of K by Na and Rb; Ti by Sn, Ge and Cr; and P by As. The KTiOPO4 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.
K0.5Na0.5TiOPO4, (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 Na0.5Rb0.5TiOPO4 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 K0.5Rb0.5TiOPO4 (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 K1-xNaxTiOPO4 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 KTiOPO4 synthesis temperature gave almost identical results (K(1):94.4(6)%Na,K(2):81.0(4)%K).
Substitution for Ti by Sn or Ge gave rise to only partial ordering. The structures of KTi0.5Sn0.5OPO4(Ti(1):63.9%Ti, Ti(2):64.6(5)%Sn) and KTi0.5Ge0.5OPO4 (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 KTiOP0.5As0.5O4(P(1):56(1)%P, P(2):56(1)%As). Measurements on KTiOPAsxO4 powders indicated that the Nd:YAG second harmonic intensity increased with As. The structures of RbTi0.5Sn0.5OPO4 and RbTiOP0.5As0.5O4 were investigated by powder X-ray diffraction and showed similar ordering to the K isomorphs.
Simultaneous substitution for both K and Ti was investigated by Rietveld analysis of combined X-ray and TOP neutron powder diffraction data on K0.5Na0.5Ti0.5Sn0.5OPO4 (K(1):73(4.)%Na,Ti(1):66.9(3)%Ti), Na0.5Rb0.5Ti0.5Sn0.5OPO4 (K(1):54(2)%Rb, Ti(1):67.0(5)%Ti), K0.5Rb0.5Ti0.5Sn0.5OPO4 (K(1):77(1)%K, Ti(1):66.1(3)%Ti) and K0.5Rb0.5SnOPO4(K(1):64(2)%K).
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.95Cr.05OPO4 isomorph, (3.2(6)%Cr on both Ti sites). Substitution of a larger amount of Cr produced K1.8Ti1.2Cr.8(PO4)3, (space group P2l3, 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).