- Email: [email protected]
Structure refinement of nonlinear optical material K0.97Ti0.97Nb0.03OPO4
August 1998
Materials Letters 36 Ž1998. 266–270
Structure refinement of nonlinear optical material K 0.97Ti 0.97 Nb 0.03 OPO4 S. Ganesa Moorthy a , F. Joseph Kumar a , C. Subramanian a , G. Bocelli b, P. Ramasamy a,) b
a Crystal Growth Centre, Anna UniÕersity, Chennai, 600 025, India Centro di Studio per la Strutturistica Diffrattometrica, UniÕersita, degli studi di Parma, Viale delle Scienze, 1-43100 Parma, Italy
Received 17 December 1997; revised 28 January 1998; accepted 30 January 1998
Abstract KTP crystals doped with niobium were grown by spontaneous crystallization using K 6 P4O 13 as the solvent. Crystal structure of K 0.97Ti 0.97 Nb 0.03 OPO4 was refined by using single crystal X-ray diffraction data. The structure is orthorhombic ˚ b s 6.403Ž2. A, ˚ c s 10.580Ž3. A. ˚ Substitution of Ti 4q by Nb 5q Žspace group Pna2 1 . with cell parameters a s 12.816Ž3. A, was charge compensated by formation of potassium vacancies in the KŽ1. site. Growth layers observed on the surface of the crystals using optical microscope indicated two-dimensional growth mechanism. q 1998 Elsevier Science B.V. All rights reserved. PACS: 78.66.Nk Keywords: Potassium titanyl phosphate; Niobium substitution; Flux method; X-ray diffraction analysis; Structure refinement; Lattice parameters; Nonlinear optical material
1. Introduction Potassium titanyl phosphate ŽKTP. is one of the best available NLO materials at present which is capable of many compositional modifications w1–5x. The KTP structure, point group mm2, is given by the chemical formula Kq4wTi 4y xOŽP 5q .O 4 , where the curly brackets indicate 9- or 8-fold coordination, the square brackets 6-fold octahedral coordination, and the parentheses 4-fold tetrahedral coordination. Several isomorphous substitutions have been made in the KTP structure with the intention of understanding the origin of its NLO behaviour. The structural )
Corresponding author. Fax: q91-44-2352774.
origin of the nonlinear optical response of KTP has long been considered to be the anomalously short Ti–O bonds w6x. Niobium is known to substitute isomorphously for Ti in a number of mixed niobate–titanate compounds w7x and an increase in the birefringence of KTP has been observed with doping of Nb which has realized SHG in the blue region w8x. Recently the phase matching angle for SHG of Nd:YAG 1.064 m m has been found to vary with the amount of Nb doping w9x and a Non Critical Phase Matching has been achieved for a Nb mole percentage of 4. The crystal structure of KTP doped with 15% Nb 2 O5 in the starting composition has been reported earlier w10x and it showed a partial replacement of Ti 4q by Nb 5q. In this compound the
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 0 5 4 - 8
S.G. Moorthy et al.r Materials Letters 36 (1998) 266–270
267
necessary charge balance occurred spontaneously by the creation of Kq vacancies. A correlation between the changes in the crystal structure of the material and in particular the short Ti–O bond distances and the optical properties was proposed. In order to clarify the correlation between the compositional modifications a study on a compound ŽKTPNb. doped with 5 mol% of Nb has been carried out.
2. Crystal growth In the Nb 5q doped KTP crystals, a bulk charge balance can be achieved by creating potassium vacancies w8x, so that a compositional formula was given as K 1y x Nb xTi 1yxOPO4 . Considering the charge balance, we have adjusted the content of potassium salt in the starting composition. Nb-doped KTiOPO4 ŽKTPNb. crystals were grown using the flux method with the addition of 5 mol% of Nb in the starting composition which substitutes the Ti. Liquid solutions of KTPNb in K 6 P4 O 13 were made by in situ reaction of TiO 2 , K 2 CO 3 , Nb 2 O5 and NH 4 H 2 PO4 . Crystals were grown in a 50 cc platinum crucible in the temperature range 950–8008C. The solutersolvent weight ratio was 0.55. The saturation temperature was determined accurately by hot stage microscopy w11x. The crystal growth experiments were carried out in a vertical tube furnace. The temperature was controlled by a Eurotherm 818P instrument. In a typical growth run 30 g of the charge was loaded into the platinum crucible. The mixture was then slowly heated Ž308Crh. till 4008C to avoid problems during melting and decomposition
Fig. 2. Layer growth pattern.
of NH 4 H 2 PO4 and also during the decomposition of K 2 CO 3 . The solution was homogenized for 24 h at 10508C and then brought to the saturation temperature. For the formation of KTP structured aggregates in the liquid phase before crystal growth, the solution was rapidly cooled below the saturation temperature at the rate of 1008Crh. Again the solution was homogenized above the saturation temperature and then cooling was commenced at the rate of 1 to 58Crh. Fig. 1 shows the crucible with the crystals nucleated on the surface of the solution. The crystals were removed from the solidified mass using hot water. The crystals were generally colourless and transparent. The crystals grew as small Ž5 mm2 . transparent colourless plates of thickness 0.5 mm. The morphology was not similar to the KTP habit. The layer growth pattern observed in one of the crystals is shown in Fig. 2 clearly indicating that the crystal grows by two-dimensional nucleation.
3. X-ray analysis
Fig. 1. Crystals nucleated on the surface of the solution.
Table 1 reports a summary of the X-ray data collection and the structure refinement procedures. The cell parameters were obtained through 56 reflections accurately well centered on the diffractometer with a procedure, which repeatedly improves the angular values to reach the maximum of intensity until the angles are moving not more than 0.018. The intensity data were collected at room temperature using a specimen mounted on a Siemens AED single crystal diffractometer equipped with a PC w12x. The absorption correction was obtained through
268
S.G. Moorthy et al.r Materials Letters 36 (1998) 266–270
Table 1 Summary of data collection and refinement procedures Crystal
colour shape size Žmm.
Symmetry Space group No of reflections for cell definition with Žrange Ž8.
˚. Cell parameters ŽA
a b c
˚ 3. Volume ŽA Z Diffractometer Radiation ˚. Lambda ŽA No of collected reflections No of observed reflections criterion for observed
u range Ž8. Indices range
h k l
Number of check reflectionsrinterval Refinement least-square method Final R factor Žall. refl.. Final R factor Žobs. refl.. Final R w factor Weighting scheme where Goodness of fit Absorption minrmax coefficient Extinction coefficient Shiftresd meanrmax Number of refined parameters Flack parameter ˚ 3. Final electron density minrmax Že A
DIFABS program w13x with the Walker and Stuart method w14x. The refinement procedure by means of the SHELX93 program w15x, starts with the data of undoped KTP structure as initial model w16x. The refinement gave an R agreement factor of about 14%. A first correction for absorption performed at this stage dropped the R value to 0.09. At this point small percentage of Nb was introduced in the Ti sites and the obtained R value was 0.087. An attempt to introduce a small percentage of Nb in the tetrahedral P site gave a dramatic worsening of the R factor.
transparent prism 0.17 = 0.25 = 0.32 orthorhombic Pna2 1 56 5.0–13.0 12.816Ž3. 6.403Ž2. 10.580Ž3. 868.2Ž4. 8 Siemens AED MoK a 0.71069 1986 1889 IF2s Ž I. 3–35 0r20 0r10 y3r16 1r100 full matrix 0.039 0.046 0.119 w s 1rw s 2 Fo2 q Ž0.087p . 2 q 0.89 p x p s wmaxŽ Fo2 ,0. q2 Fc2 xr3 1.164 0.77r1.58 0.101 0.027ry 0.261 149 0.0015 y1.37r1.19
The subsequent step was consistent with a release of the occupancy values of the KŽ1. atoms while the KŽ2. remained unitary, KŽ1. showed a value of 0.97 indicating some vacancies of K atoms in this site. So it seems differently from KTNP w10x in which both the K atoms showed vacancies. A further absorption correction was then carried out using the preceding method and on the basis of the resulting structure. After the anisotropic refinement of all atoms, the attempt to produce a small variation of the Nb content was insensitive on the atomic coordinates and on SOF value both of Ti and K atoms. The
S.G. Moorthy et al.r Materials Letters 36 (1998) 266–270 Table 2 Fractional atomic coordinates Ž=10 4 . and occupancy values
K1 K2 Ti1 Nb1 Ti2 Nb2 P1 P2 O1 O2 O3 O4 OT1 OT2 O5 O6 O7 O8
xra
yr b
zrc
s.o.f
3782 Ž9. 1050 Ž8. 3728 Ž4.
7809 Ž14. 6982 Ž18. 5000 Ž8.
3097 Ž13. 652 Ž12. y5 Ž6.
2470 Ž4.
2684 Ž8.
2501 Ž6.
4981 Ž6. 1812 Ž6. 4861 Ž24. 5097 Ž22. 4001 Ž20. 5940 Ž18. 2248 Ž23. 2240 Ž23. 1125 Ž21. 1116 Ž22. 2529 Ž23. 2534 Ž23.
3369 Ž12. 5017 Ž12. 4850 Ž48. 4664 Ž54. 1995 Ž43. 1938 Ž41. 9655 Ž47. 407 Ž48. 3107 Ž40. 6919 Ž42. 5397 Ž53. 4607 Ž51.
2580 Ž10. 5105 Ž9. 1473 Ž28. 3811 Ž26. 2775 Ž27. 2391 Ž27. 6409 Ž26. 3880 Ž25. 5395 Ž27. 4850 Ž29. 6260 Ž27. 3974 Ž27.
0.9756 1.0 0.9421 0.0579 0.9920 0.0080 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
fractional atomic coordinates of atoms with their SOF values are in Table 2.
269
˚ and 0.071 A, ˚ respecwŽ S i6 Ž x i y x m .r6x are 0.084 A tively. These values are both significantly smaller ˚ x conthan those found in KTP w0.143 and 0.122 A firming, as suggested for KTNP, that an Nb content in Ti sites tends to reduce the octahedral distortion. The difference between the structure of the KTNP w10x and KTPNb materials consists in the occupancy values of the K sites. In fact, in the first material an equal loss of potassium was found in both KŽ1. and KŽ2. sites while in our case, loss was found only in the KŽ1. site. 5. Conclusion KTP crystals doped with Nb were grown by flux method. The structure of KTPNb crystal was refined well. For a concentration of 5 mol% of Nb in the starting composition about 3.5% was found to be incorporated in the crystal lattice. The Nb atoms were found to occupy the Ti site and the charge compensation was found to occur only in the KŽ1. site, in contrast to the earlier reported result of occupancy in both the KŽ1. and KŽ2. sites.
4. Geometry As observed in KTNP w10x, the Nb atoms enter preferentially in the TiŽ1. sites with a total of about 3.5%. In our case this preference is significantly marked. The bond lengths in the TiŽ1. –O6 octahe˚ and from dron range from 1.731Ž3. to 2.137Ž3. A ˚ 1.749Ž3. to 2.083Ž3. A in the TiŽ2. –O6 octahedron ˚ and 1.969Ž43. A, ˚ with mean values of 1.972Ž55. A respectively. These values agree very well with those already found in KTP and KTNP ŽTable 3.. However the distortion of the octahedra measured as the averaged deviations from the means of Ti–O bond lengths Table 3 Comparison between mean geometrical parameters
˚. Bond lengths ŽA Ti1–O Ti2–O K1–O K2–O
KTP
KTNP
KTPNb
1.972Ž90. 1.967Ž79. 2.845Ž111. 2.934Ž127.
1.973Ž87. 1.967Ž63. 2.854Ž124. 2.943Ž133.
1.972Ž55. 1.969Ž43. 2.845Ž48. 2.930Ž50.
Acknowledgements One of the authors ŽSGM. is grateful to the Council of Scientific and Industrial Research for the award of Senior Research Fellowship.
References w1x R. Masse, J. Grenier, Bull. Soc. Fr. Kineral. Cristallofr. 94 Ž1971. 437. w2x C.E. Bamberger, G.M. Begun, O.B. Cavin, J. Solid State Chem. 73 Ž1988. 317. w3x N.S. Slobodanyik, P.G. Nagornyi, V.V. Skopenko, E.S. Lugovskaya, Russian J. Inorg. Chem. 32 Ž1987. 1023. w4x M. El Brahimi, J. Durand, Rev. de Chimie Minerale 23 Ž3. Ž1987. 720. w5x J. Gopalakrishnan, K. Kasthuri Rangan, B. Raghavendra Prasad, C.K. Subramanian, J. Solid State Chem. 111 Ž1994. 41. w6x Tordjman, R. Masse, J. Guitel, Z. Krist. 139 Ž1974. 103. w7x A.D. Wadsley, Acta Cryst. 17 Ž1964. 623. w8x L.T. Cheng, L.K. Cheng, R.L. Harlow, J.D. Berlein, Appl. Phys. Lett. 64 Ž1994. 155.
270
S.G. Moorthy et al.r Materials Letters 36 (1998) 266–270
w9x W. Jing-qian, W. Ji-Yang, L. Yao-gang, W. Chang-ging, S. Zong-shu, G. Qing-cai, J. Min-Hua, Chin. Phys. Lett. 13 Ž1996. 203. w10x P.A. Thomas, B.E. Watts, Solid State Commun. 73 Ž1990. 97. w11x F.J. Kumar, S.G. Moorthy, D. Jayaraman, C. Subramanian, J. Crystal Growth 160 Ž1996. 129. w12x D. Belletti, A. Cantoni, G. Pasquinelli, Gesione on Line di
w13x w14x w15x w16x
Diffrattometro a Cristallo Singolo Siemens AED con PC, CSSD Parma, Italy, Internal Report, 1991, pp. 1–91. P. Gluzinski, Set of Programs for the X-ray Structural Calculations, IChO, Polish Acad. Sci., Warszawa, Poland Ž1989.. N. Walker, D. Stuart, Acta Cryst. A 39 Ž1983. 158. G. Sheldrick, SHELX 93 Ž1993. . P.A. Thomas, A.M. Glazer, B.E. Watts, Acta. Cryst. B 46 Ž1990. 333.
Materials Letters 36 Ž1998. 266–270
Structure refinement of nonlinear optical material K 0.97Ti 0.97 Nb 0.03 OPO4 S. Ganesa Moorthy a , F. Joseph Kumar a , C. Subramanian a , G. Bocelli b, P. Ramasamy a,) b
a Crystal Growth Centre, Anna UniÕersity, Chennai, 600 025, India Centro di Studio per la Strutturistica Diffrattometrica, UniÕersita, degli studi di Parma, Viale delle Scienze, 1-43100 Parma, Italy
Received 17 December 1997; revised 28 January 1998; accepted 30 January 1998
Abstract KTP crystals doped with niobium were grown by spontaneous crystallization using K 6 P4O 13 as the solvent. Crystal structure of K 0.97Ti 0.97 Nb 0.03 OPO4 was refined by using single crystal X-ray diffraction data. The structure is orthorhombic ˚ b s 6.403Ž2. A, ˚ c s 10.580Ž3. A. ˚ Substitution of Ti 4q by Nb 5q Žspace group Pna2 1 . with cell parameters a s 12.816Ž3. A, was charge compensated by formation of potassium vacancies in the KŽ1. site. Growth layers observed on the surface of the crystals using optical microscope indicated two-dimensional growth mechanism. q 1998 Elsevier Science B.V. All rights reserved. PACS: 78.66.Nk Keywords: Potassium titanyl phosphate; Niobium substitution; Flux method; X-ray diffraction analysis; Structure refinement; Lattice parameters; Nonlinear optical material
1. Introduction Potassium titanyl phosphate ŽKTP. is one of the best available NLO materials at present which is capable of many compositional modifications w1–5x. The KTP structure, point group mm2, is given by the chemical formula Kq4wTi 4y xOŽP 5q .O 4 , where the curly brackets indicate 9- or 8-fold coordination, the square brackets 6-fold octahedral coordination, and the parentheses 4-fold tetrahedral coordination. Several isomorphous substitutions have been made in the KTP structure with the intention of understanding the origin of its NLO behaviour. The structural )
Corresponding author. Fax: q91-44-2352774.
origin of the nonlinear optical response of KTP has long been considered to be the anomalously short Ti–O bonds w6x. Niobium is known to substitute isomorphously for Ti in a number of mixed niobate–titanate compounds w7x and an increase in the birefringence of KTP has been observed with doping of Nb which has realized SHG in the blue region w8x. Recently the phase matching angle for SHG of Nd:YAG 1.064 m m has been found to vary with the amount of Nb doping w9x and a Non Critical Phase Matching has been achieved for a Nb mole percentage of 4. The crystal structure of KTP doped with 15% Nb 2 O5 in the starting composition has been reported earlier w10x and it showed a partial replacement of Ti 4q by Nb 5q. In this compound the
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 0 5 4 - 8
S.G. Moorthy et al.r Materials Letters 36 (1998) 266–270
267
necessary charge balance occurred spontaneously by the creation of Kq vacancies. A correlation between the changes in the crystal structure of the material and in particular the short Ti–O bond distances and the optical properties was proposed. In order to clarify the correlation between the compositional modifications a study on a compound ŽKTPNb. doped with 5 mol% of Nb has been carried out.
2. Crystal growth In the Nb 5q doped KTP crystals, a bulk charge balance can be achieved by creating potassium vacancies w8x, so that a compositional formula was given as K 1y x Nb xTi 1yxOPO4 . Considering the charge balance, we have adjusted the content of potassium salt in the starting composition. Nb-doped KTiOPO4 ŽKTPNb. crystals were grown using the flux method with the addition of 5 mol% of Nb in the starting composition which substitutes the Ti. Liquid solutions of KTPNb in K 6 P4 O 13 were made by in situ reaction of TiO 2 , K 2 CO 3 , Nb 2 O5 and NH 4 H 2 PO4 . Crystals were grown in a 50 cc platinum crucible in the temperature range 950–8008C. The solutersolvent weight ratio was 0.55. The saturation temperature was determined accurately by hot stage microscopy w11x. The crystal growth experiments were carried out in a vertical tube furnace. The temperature was controlled by a Eurotherm 818P instrument. In a typical growth run 30 g of the charge was loaded into the platinum crucible. The mixture was then slowly heated Ž308Crh. till 4008C to avoid problems during melting and decomposition
Fig. 2. Layer growth pattern.
of NH 4 H 2 PO4 and also during the decomposition of K 2 CO 3 . The solution was homogenized for 24 h at 10508C and then brought to the saturation temperature. For the formation of KTP structured aggregates in the liquid phase before crystal growth, the solution was rapidly cooled below the saturation temperature at the rate of 1008Crh. Again the solution was homogenized above the saturation temperature and then cooling was commenced at the rate of 1 to 58Crh. Fig. 1 shows the crucible with the crystals nucleated on the surface of the solution. The crystals were removed from the solidified mass using hot water. The crystals were generally colourless and transparent. The crystals grew as small Ž5 mm2 . transparent colourless plates of thickness 0.5 mm. The morphology was not similar to the KTP habit. The layer growth pattern observed in one of the crystals is shown in Fig. 2 clearly indicating that the crystal grows by two-dimensional nucleation.
3. X-ray analysis
Fig. 1. Crystals nucleated on the surface of the solution.
Table 1 reports a summary of the X-ray data collection and the structure refinement procedures. The cell parameters were obtained through 56 reflections accurately well centered on the diffractometer with a procedure, which repeatedly improves the angular values to reach the maximum of intensity until the angles are moving not more than 0.018. The intensity data were collected at room temperature using a specimen mounted on a Siemens AED single crystal diffractometer equipped with a PC w12x. The absorption correction was obtained through
268
S.G. Moorthy et al.r Materials Letters 36 (1998) 266–270
Table 1 Summary of data collection and refinement procedures Crystal
colour shape size Žmm.
Symmetry Space group No of reflections for cell definition with Žrange Ž8.
˚. Cell parameters ŽA
a b c
˚ 3. Volume ŽA Z Diffractometer Radiation ˚. Lambda ŽA No of collected reflections No of observed reflections criterion for observed
u range Ž8. Indices range
h k l
Number of check reflectionsrinterval Refinement least-square method Final R factor Žall. refl.. Final R factor Žobs. refl.. Final R w factor Weighting scheme where Goodness of fit Absorption minrmax coefficient Extinction coefficient Shiftresd meanrmax Number of refined parameters Flack parameter ˚ 3. Final electron density minrmax Že A
DIFABS program w13x with the Walker and Stuart method w14x. The refinement procedure by means of the SHELX93 program w15x, starts with the data of undoped KTP structure as initial model w16x. The refinement gave an R agreement factor of about 14%. A first correction for absorption performed at this stage dropped the R value to 0.09. At this point small percentage of Nb was introduced in the Ti sites and the obtained R value was 0.087. An attempt to introduce a small percentage of Nb in the tetrahedral P site gave a dramatic worsening of the R factor.
transparent prism 0.17 = 0.25 = 0.32 orthorhombic Pna2 1 56 5.0–13.0 12.816Ž3. 6.403Ž2. 10.580Ž3. 868.2Ž4. 8 Siemens AED MoK a 0.71069 1986 1889 IF2s Ž I. 3–35 0r20 0r10 y3r16 1r100 full matrix 0.039 0.046 0.119 w s 1rw s 2 Fo2 q Ž0.087p . 2 q 0.89 p x p s wmaxŽ Fo2 ,0. q2 Fc2 xr3 1.164 0.77r1.58 0.101 0.027ry 0.261 149 0.0015 y1.37r1.19
The subsequent step was consistent with a release of the occupancy values of the KŽ1. atoms while the KŽ2. remained unitary, KŽ1. showed a value of 0.97 indicating some vacancies of K atoms in this site. So it seems differently from KTNP w10x in which both the K atoms showed vacancies. A further absorption correction was then carried out using the preceding method and on the basis of the resulting structure. After the anisotropic refinement of all atoms, the attempt to produce a small variation of the Nb content was insensitive on the atomic coordinates and on SOF value both of Ti and K atoms. The
S.G. Moorthy et al.r Materials Letters 36 (1998) 266–270 Table 2 Fractional atomic coordinates Ž=10 4 . and occupancy values
K1 K2 Ti1 Nb1 Ti2 Nb2 P1 P2 O1 O2 O3 O4 OT1 OT2 O5 O6 O7 O8
xra
yr b
zrc
s.o.f
3782 Ž9. 1050 Ž8. 3728 Ž4.
7809 Ž14. 6982 Ž18. 5000 Ž8.
3097 Ž13. 652 Ž12. y5 Ž6.
2470 Ž4.
2684 Ž8.
2501 Ž6.
4981 Ž6. 1812 Ž6. 4861 Ž24. 5097 Ž22. 4001 Ž20. 5940 Ž18. 2248 Ž23. 2240 Ž23. 1125 Ž21. 1116 Ž22. 2529 Ž23. 2534 Ž23.
3369 Ž12. 5017 Ž12. 4850 Ž48. 4664 Ž54. 1995 Ž43. 1938 Ž41. 9655 Ž47. 407 Ž48. 3107 Ž40. 6919 Ž42. 5397 Ž53. 4607 Ž51.
2580 Ž10. 5105 Ž9. 1473 Ž28. 3811 Ž26. 2775 Ž27. 2391 Ž27. 6409 Ž26. 3880 Ž25. 5395 Ž27. 4850 Ž29. 6260 Ž27. 3974 Ž27.
0.9756 1.0 0.9421 0.0579 0.9920 0.0080 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
fractional atomic coordinates of atoms with their SOF values are in Table 2.
269
˚ and 0.071 A, ˚ respecwŽ S i6 Ž x i y x m .r6x are 0.084 A tively. These values are both significantly smaller ˚ x conthan those found in KTP w0.143 and 0.122 A firming, as suggested for KTNP, that an Nb content in Ti sites tends to reduce the octahedral distortion. The difference between the structure of the KTNP w10x and KTPNb materials consists in the occupancy values of the K sites. In fact, in the first material an equal loss of potassium was found in both KŽ1. and KŽ2. sites while in our case, loss was found only in the KŽ1. site. 5. Conclusion KTP crystals doped with Nb were grown by flux method. The structure of KTPNb crystal was refined well. For a concentration of 5 mol% of Nb in the starting composition about 3.5% was found to be incorporated in the crystal lattice. The Nb atoms were found to occupy the Ti site and the charge compensation was found to occur only in the KŽ1. site, in contrast to the earlier reported result of occupancy in both the KŽ1. and KŽ2. sites.
4. Geometry As observed in KTNP w10x, the Nb atoms enter preferentially in the TiŽ1. sites with a total of about 3.5%. In our case this preference is significantly marked. The bond lengths in the TiŽ1. –O6 octahe˚ and from dron range from 1.731Ž3. to 2.137Ž3. A ˚ 1.749Ž3. to 2.083Ž3. A in the TiŽ2. –O6 octahedron ˚ and 1.969Ž43. A, ˚ with mean values of 1.972Ž55. A respectively. These values agree very well with those already found in KTP and KTNP ŽTable 3.. However the distortion of the octahedra measured as the averaged deviations from the means of Ti–O bond lengths Table 3 Comparison between mean geometrical parameters
˚. Bond lengths ŽA Ti1–O Ti2–O K1–O K2–O
KTP
KTNP
KTPNb
1.972Ž90. 1.967Ž79. 2.845Ž111. 2.934Ž127.
1.973Ž87. 1.967Ž63. 2.854Ž124. 2.943Ž133.
1.972Ž55. 1.969Ž43. 2.845Ž48. 2.930Ž50.
Acknowledgements One of the authors ŽSGM. is grateful to the Council of Scientific and Industrial Research for the award of Senior Research Fellowship.
References w1x R. Masse, J. Grenier, Bull. Soc. Fr. Kineral. Cristallofr. 94 Ž1971. 437. w2x C.E. Bamberger, G.M. Begun, O.B. Cavin, J. Solid State Chem. 73 Ž1988. 317. w3x N.S. Slobodanyik, P.G. Nagornyi, V.V. Skopenko, E.S. Lugovskaya, Russian J. Inorg. Chem. 32 Ž1987. 1023. w4x M. El Brahimi, J. Durand, Rev. de Chimie Minerale 23 Ž3. Ž1987. 720. w5x J. Gopalakrishnan, K. Kasthuri Rangan, B. Raghavendra Prasad, C.K. Subramanian, J. Solid State Chem. 111 Ž1994. 41. w6x Tordjman, R. Masse, J. Guitel, Z. Krist. 139 Ž1974. 103. w7x A.D. Wadsley, Acta Cryst. 17 Ž1964. 623. w8x L.T. Cheng, L.K. Cheng, R.L. Harlow, J.D. Berlein, Appl. Phys. Lett. 64 Ž1994. 155.
270
S.G. Moorthy et al.r Materials Letters 36 (1998) 266–270
w9x W. Jing-qian, W. Ji-Yang, L. Yao-gang, W. Chang-ging, S. Zong-shu, G. Qing-cai, J. Min-Hua, Chin. Phys. Lett. 13 Ž1996. 203. w10x P.A. Thomas, B.E. Watts, Solid State Commun. 73 Ž1990. 97. w11x F.J. Kumar, S.G. Moorthy, D. Jayaraman, C. Subramanian, J. Crystal Growth 160 Ž1996. 129. w12x D. Belletti, A. Cantoni, G. Pasquinelli, Gesione on Line di
w13x w14x w15x w16x
Diffrattometro a Cristallo Singolo Siemens AED con PC, CSSD Parma, Italy, Internal Report, 1991, pp. 1–91. P. Gluzinski, Set of Programs for the X-ray Structural Calculations, IChO, Polish Acad. Sci., Warszawa, Poland Ž1989.. N. Walker, D. Stuart, Acta Cryst. A 39 Ž1983. 158. G. Sheldrick, SHELX 93 Ž1993. . P.A. Thomas, A.M. Glazer, B.E. Watts, Acta. Cryst. B 46 Ž1990. 333.