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Synthesis, crystal structure and superconductivity of LaNiPO
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Solid State Sciences 10 (2008) 193e197 www.elsevier.com/locate/ssscie
Synthesis, crystal structure and superconductivity of LaNiPO Marcus Tegel, Daniel Bichler, Dirk Johrendt* Department Chemie und Biochemie der Ludwig-Maximilians-Universita¨t Mu¨nchen, Butenandtstrasse 5-13 (Haus D), 81377 Mu¨nchen, Germany Received 16 August 2007; received in revised form 22 August 2007; accepted 27 August 2007 Available online 2 September 2007
Abstract Single crystals of LaNiPO were synthesized by reacting La, P and NiO at 1173 K in a tin flux under argon atmosphere. A phase analysis and crystal structure determination was carried out using X-ray powder and single crystal methods. The quaternary phosphide oxide crystallizes in the tetragonal ZrCuSiAs structure (P4/nmm, a ¼ 404.53(1) pm, c ¼ 810.54(3) pm, Z ¼ 2), which is characterized by layers of edge-sharing 2 2 N ½La4=4 O and N ½NiP4=4 tetrahedra alternating along [001]. LaNiPO is a Pauli paramagnetic metal at room temperature and becomes a type-II superconductor at TC ¼ 4.3 K with an upper critical field Hc2 z 320 mT at 1.8 K. Connections between the crystal chemistry and the occurrence of superconductivity in LaNiPO and related ThCr2Si2 type compounds are discussed. Ó 2007 Elsevier Masson SAS. All rights reserved. Keywords: Phosphide oxides; Lanthanum; Nickel; Crystal structure; Superconductivity
1. Introduction Quaternary rare earth transition metal phosphide oxides with the tetragonal ZrCuSiAs structure [1] are known for more than ten years [2]. ZrCuSiAs was first described as a filled PbFCl type; silicon and copper occupy the tetrahedral holes and arsenic occupies the octahedral holes in an fcc packing of zirconium. But the two-dimensional order of the atoms in the tetrahedral holes leads to a more layer-like structure, thus also the related phosphide oxides RMPO (R ¼ rare earth metal, M ¼ transition metal) are better described as alternating layers of edge-sharing MP4/4 and R4/4O tetrahedra. Among the iron group compounds, the series RFePO (R ¼ LaeNd, Sm, Gd), RCoPO (R ¼ LaeNd, Sm) and RRuPO (R ¼ LaeNd, Sm, Gd) are known to date [2], as well as the manganese phosphide oxides RMnPO (R ¼ LaeNd, Sm, GdeDy) [3]. Recently, dimorphic CeZnPO and PrZnPO were reported [4], forming either the tetragonal ZrCuSiAs or the rhombohedral NdZnPO type, depending on the synthesis conditions [5].
Even though numerous RMPO compounds have been synthesized and characterized, virtually nothing is known about their electronic and magnetic properties. Magnetic data are only reported for the antimonide oxides CeZnSbO and PrZnSbO [6]. But beyond the typically localized magnetism of the trivalent rare earth ions, especially phosphide oxides with open-shell 3d-metals may display interesting magnetic and electronic phenomena. First motivating results are the antiferromagnetic order at room temperature found in the ZrCuSiAs type phosphide fluoride BaMnPF [7] and the recently reported superconductivity of LaFePO [8,9]. We have started systematic investigations about the structureeproperty relationships of quaternary phosphide oxides with 3d-transition metals with particular attention to magnetic and electronic phenomena. In the present paper, we report on the synthesis, crystal structure and superconductivity of the first nickel phosphide oxide LaNiPO. 2. Experimental 2.1. Synthesis
* Corresponding author. Tel.: þ49 (0)89 2180 77430; fax: þ49 (0)89 2180 77431. E-mail address: [email protected] (D. Johrendt). 1293-2558/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2007.08.016
Single crystals of LaNiPO were prepared by heating a mixture of 375.0 mg La (99.9%, Smart Elements), 201.7 mg
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M. Tegel et al. / Solid State Sciences 10 (2008) 193e197
NiO (99.99%, Sigma-Aldrich) and 83.6 mg P (red, 99þ%, Sigma-Aldrich) with 2000 mg Sn (99.99%, Alfa Aesar) in an alumina crucible, which was sealed in a silica tube under an atmosphere of purified argon. The sample was heated to 1173 K at a rate of 40 K/h, kept at this temperature for 10 days and slowly cooled down to room temperature at a rate of 3 K/h. The crucible was smashed and the tin bar dissolved in 6 M HCl at room temperature. The remaining sample consisted of single crystals of LaNiPO beside small amounts of LaNi2P2 (7%), Ni2SnP (4%) and Ni3Sn4 (<1%). Further attempts to optimize the synthesis conditions with regard to reaction temperature or duration were unsuccessful. Samples prepared directly from the starting material without tin flux yielded only small amounts of LaNiPO besides LaNi2P2 as main product. 2.2. Characterization X-ray powder patterns of finely ground samples were recorded on a Stoe Stadi-P diffractometer (Mo-Ka1 radiation, Ge(111)-monochromator, l ¼ 70.93 pm, 7 PSD-detector, silicon as external standard) at room temperature. The powder data was analyzed by the Rietveld method using the GSAS suite [10]. Single crystal data were collected in oscillation mode using an Enraf-Nonius k-CCD, equipped with a rotating anode (Mo-Ka radiation, graphite-monochromator, l ¼ 71.073 pm). The intensities were corrected for absorption with SADABS [11] and refined against F2 using SHELXL97 [12] with anisotropic displacement parameters for all atoms. Refinements of the site occupation factors converged to full occupation of all sites within standard deviations. Starting parameters were taken from isostructural PrFePO [2]. 2.3. Magnetic measurements Magnetization measurements were performed using a Quantum-Design SQUID magnetometer (MPMS-XL5) between 1.8 and 300 K with magnetic flux densities up to 5 T. A cold pressed pellet of LaNiPO was put into a gelatin capsule and fixed in a straw as sample holder. Zero-field-cooling (Shielding) and field-cooling (Meissner) measurement cycles were performed at 1 mT between 1.8 and 8 K in DC mode. The magnetic susceptibility between 2 and 300 K was measured at 1 T. The magnetization isotherm at 8 K is linear up to 5 T and reveals only small traces of ferromagnetic contamination. 2.4. Electric resistivity measurements The electric resistivity of a LaNiPO sample was measured using a DC 4-point current reversal method [13]. A cold pressed pellet was mounted onto a special sample holder with Apiezon-N grease, contacted to four copper wires with silver conducting paint and inserted into the cryogenic system of the SQUID magnetometer. The voltage drop along the sample at a constant current of 10 mA was measured with a Keithley nanovoltmeter between 1.8 and 8 K in steps of 0.2 K.
3. Results The structure determination of a LaNiPO single crystal confirmed the tetragonal ZrCuSiAs structure (space group P4/nmm), which is isotypic to LaFePO and other related phosphide oxides. The new compound represents the first nickel phosphide oxide of this structural family. Crystallographic data and selected bond lengths and bond angles are listed in Tables 1 and 2. Further details about the crystal structure determination may be obtained from: Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany, e-mail: [email protected], on quoting the registration No. CSD-391427. In order to detect and quantify significant impurity phases, we refined the X-ray powder pattern of a sample by the Rietveld method using the single crystal data of LaNiPO as starting parameters. The experimental pattern as shown in Fig. 1 was fitted using three phases, namely 89.2% LaNiPO, 6.9% LaNi2P2 and 3.9% Ni2SnP. Additional small traces (<1%) of Ni3Sn4 were also detected, but not included in the Rietveld fit. Thus, the synthesis yielded about 90% LaNiPO and the closely related LaNi2P2 as the main byproduct. The crystal structure of LaNiPO consists of layers of edgesharing La4/4O and NiP4/4 tetrahedra alternating along [001] as depicted in Fig. 2. The NieP distance is 226.5(2) pm, i.e. Table 1 LaNiPO: crystallographic data and details of the structure refinement Crystal system Space group Lattice constant a (pm) Lattice constant c (pm) Unit cell determination method Formula units (Z ) Calculated density Dx (g/cm3) Molar volume Vm (cm3/mol) Crystal dimensions (mm3) Diffractometer and wavelength hmax ¼ kmax lmax qmax ( ) F(000) Absorption coefficient m (mm1) Data reduction and absorption correction Number of refined parameters Reflections (unique) Rint/Rs Reflections with jIj 2s(I ) Structure determination and refinement Scattering factors R1/R1 with jIj 2s(I ) wR2/goodness of fit (GooF) Residual density, max (e 106 pm3) Residual density, min (e 106 pm3)
Tetragonal P4/nmm (no. 129, origin choice 2) 404.53(1) 810.54(3) Rietveld fit 2 6.124 39.94 0.05 0.05 0.02 Enraf-Nonius k-CCD, Mo-Ka: l ¼ 71.073 pm 5 10 27.29 216 23.43 SCALEPACK [24], SADABS [11] 11 2366 (117) 0.0456/0.0220 114 Empirical/SHELXL-97 [12] International Tables, vol. C 0.0274/0.0269 0.0713/1.272 2.572 0.802
M. Tegel et al. / Solid State Sciences 10 (2008) 193e197 Table 2 Atomic parameters, anisotropic displacement factors and selected bond lengths (pm) and angles ( ) for LaNiPO (Uij in pm2; U11 ¼ U22; U12 ¼ U13 ¼ U23 ¼ 0) Atom
Wyck.
x/a
y/b
z/c
U11
U33
Ueq
La Ni P O
(2c) (2b) (2c) (2a)
1/4 3/4 1/4 3/4
1/4 1/4 1/4 1/4
0.15190(9) 1/2 0.6257(5) 0
87(4) 179(7) 134(13) 100(30)
116(5) 147(8) 164(17) 100(40)
97(4) 168(5) 144(9) 101(19)
LaeO LaeP LaeNi NieP NieNi
236.79(4) 338.1(2) 347.2(1) 226.5(2) 286.05(1)
(4) (4) (4) (4) (4)
LaeOeLa LaeOeLa PeNieP PeNieP
105.69(1) 117.34(3) 101.7(1) 126.5(2)
close to the sum of the covalent radii of 225 pm. As it can be recognized by PeNieP angles as large as 126.5(2) , the NiP4 tetrahedra are significantly compressed along [001]. This holds true to a smaller extent for the OLa4 tetrahedra with LaeOeLa angles of 117.3(1) and a LaeO distance of 236.8(1) pm. Lanthanum is eightfold coordinated by four oxygen and four phosphorus atoms (LaeP 338.1(2) pm), forming a distorted tetragonal antiprism. The coordination sphere of nickel is completed by four neighboring nickel atoms within the same layer at a distance of 286.1(1) pm, this can be considered as weak NieNi bonds at best. A remarkably small c/a ratio of LaNiPO (2.00) in comparison with LaFePO (2.15) and LaCoPO (2.11) results from the compression of the tetrahedra mentioned above. We believe this compression is a consequence of a weaker metalemetal bonding between the nickel atoms within the phosphide layers. The scenario is very similar among the closely related ThCr2Si2 type compounds, where metalemetal bonding plays
195
an important role. As we have shown earlier [14], an increasing electron count along the series RM2P2 (R ¼ rare earth metal, M ¼ Fe, Co, Ni) fills antibonding bands of strongly overlapping 3dxy orbitals and weakens the metalemetal bonds within the phosphide layer. As the MeM bonds become weaker and longer, the layer is expanded in the (ab)-plane and compressed perpendicular (along [001]) in order to avoid longer MeP bonds. In the case of the ThCr2Si2 type phosphides, this mechanism decreases the distance between the MP4/4 layers and can lead to the formation of short PeP bonds between adjacent layers. This can be observed in LaNi2P2 [15], which has short PeP bonds (239 pm), whereas no such bonds are formed in LaCo2P2 and LaFe2P2 [16], because the latter two have fewer antibonding electrons and therefore stronger metalemetal bonds. In LaNiPO, the NiP4/4 sheets are separated by OLa4/4 layers and therefore direct PeP bonds between the layers are impossible. On the other hand, the electron count within the NiP4/4 layer of LaNiPO is smaller than the electron count in LaNi2P2. Assuming La3þ in both cases, only one electron is transferred to the phosphide layer in LaNiPO according to (LaO)þ(NiP) but 1.5 electrons are transferred in La3þ(Ni2P2)3. Therefore the occupation of the NieNi antibonding bands is expected to be smaller in LaNiPO than in LaNi2P2 and the phosphide layer is less destabilized in the phosphide oxide. First magnetic measurements of a LaNiPO sample showed weak paramagnetism (cmol z 5 104 cm3/mol), varying only slightly with temperature, as it is common for metallic materials. Magnetization isotherms showed linear behavior but indicated small traces of an unknown ferromagnetic contamination equivalent to w10 ppm nickel metal; therefore we are unable to quantify the Pauli paramagnetism.
Fig. 1. X-ray powder pattern (þ) and Rietveld fit () of the LaNiPO sample. Refined phases are LaNiPO (89.2%), LaNi2P2 (6.9%) and Ni2SnP (3.9%). The residual intensities are caused by traces of Ni3Sn4, which was not included in the Rietveld fit.
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Fig. 2. Crystal structure of LaNiPO.
The sample, which was cooled down to 1.8 K in a zero magnetic field (zfc), showed very strong diamagnetism as it is typical of superconductors. Subsequent shielding (zfc) and Meissner (fc) measurements confirmed the superconducting transition of LaNiPO at TC ¼ 4.3 K (Fig. 3). The volume susceptibility of the sample reaches about 70% of the ideal diamagnetic shielding (c ¼ 1) at 1.8 K, therefore it is obvious that the superconductivity is not caused by an impurity phase. We also performed preliminary measurements of the critical flux densities. LaNiPO is a type-II superconductor with small critical flux densities of Bc1 z 3 mT and Bc2 z 320 mT and a calculated thermodynamic critical flux density Bc,th z 32 mT at 1.8 K. Fig. 3 also shows the electrical resistance of the sample between 1.8 and 8 K. Due to the rather arbitrary fixed contacts and an irregular shaped sample, we are unable to give specific resistivity values, but the resistance was about 100 mU at 8 K. Nevertheless, the superconducting transition at 4.3 K is clearly discernible and the resistance drops between 4.3 and 1.8 K to 3% of the value at 8 K. The critical temperature found here is in excellent agreement with the magnetization measurements (dotted line in Fig. 3). The rather broad transition (within w2 K) may be a hint at granular superconductivity or inhomogeneities of the sample. 4. Discussion LaNiPO is the first nickel phosphide oxide and the second superconducting compound in this family. Thus, the superconductivity of LaFePO does not seem to be a unique observation in this class of compounds and it is probable that superconductivity will occur frequently here. We especially want to indicate the relationship between LaNiPO and the ThCr2Si2 type compounds, where superconductivity is extremely rare with respect to the large number of about 700 compounds of this type [17]. Superconductivity has only been observed in
Fig. 3. Magnetic susceptibility (B ¼ 1 mT) and electrical resistance (I ¼ 10 mA) of the LaNiPO sample between 1.8 and 8 K.
LaIr2Ge2 (TC ¼ 1.5 K) [18], LaRu2P2 (TC ¼ 4.1 K) [19] and YIr2xSi2þx (TC ¼ 2.8e3.05 K) [20]. Contrary to this, the ThCr2Si2 type related boride-carbides RNi2B2C (R ¼ Y, Tm, Er, Ho, Lu) are well-known superconductors with critical temperatures up to 16 K for R ¼ Lu [21]. We have discussed earlier [14] that superconductivity in ThCr2Si2 type and related compounds like LuNi2B2C is associated with interactions between the transition metal atoms, which generate a peak in the density-of-states (DOS) close to the Fermi level. Very recently it was shown that superconducting LaFePO shows the same property in its electronic structure [9]. We assume that the superconductivity of LaRu2P2, LuNi2B2C, LaFePO and LaNiPO originates in the transition metal layers (RuP, NiB, FeP and NiP), which provide the necessary, but not sufficient electronic conditions according to the flat band/steep band scenario proposed by Simon [22]. We also believe that the separating layers play an important role for superconductivity. Fig. 4 shows the structures of LaRu2P2, LuNi2B2C and LaNiPO in comparison. In ThCr2Si2 type compounds like LaRu2P2 or LaNi2P2, where RuP or NiP layers are separated by lanthanum atoms, superconductivity occurs rarely and at low temperatures. But if the analogous NiB layers are separated by LuC, as in LuNi2B2C, superconductivity with relatively high TC
M. Tegel et al. / Solid State Sciences 10 (2008) 193e197
197
Fig. 4. Crystal structures of the superconductors LaRu2P2 (TC ¼ 4.1 K), LuNi2B2C (TC ¼ 16 K) and LaNiPO (TC ¼ 4.5 K).
emerges. Accordingly, LaNi2P2 and LaFe2P2 are normal metals, but if the NiP or FeP layers are sandwiched between LaO layers, as it is the case for LaNiPO and LaFePO, both become superconducting at TC ¼ 4.3 and 6.5 K, respectively [23]. These ideas open up interesting prospects of synthesizing further transition metal phosphide oxides with different separating layers in order to study the superconductivity in this class of materials in more detail. During the reviewing process, we took notice of a recently published article on LaNiPO [25]. Therein, a superconducting transition temperature of w3 K is reported in contrast to our data showing TC ¼ 4.3 K. The superconducting volume fraction of 38% (cg ¼ 5.3 103 emu/g at 1.8 K) is considerably smaller than that of our sample, which shows 73% superconducting phase (cg ¼ 9.5 102 emu/g at 1.8 K). We believe that the reasons for these differences are small amounts of ferromagnetic contaminations of the samples, which we also suspect in the case of LaFePO [23]. References [1] V. Johnson, W. Jeitschko, J. Solid State Chem. 11 (1974) 161e166. [2] B.I. Zimmer, W. Jeitschko, J.H. Albering, R. Glaum, M. Reehuis, J. Alloys Compd. 229 (1995) 238e242. [3] A.T. Nientiedt, W. Jeitschko, P.G. Pollmeier, M. Brylak, Z. Naturforsch., B: Chem. Sci. 52 (1997) 560e564. [4] H. Lincke, T. Nilges, R. Po¨ttgen, Z. Anorg. Allg. Chem. 632 (2006) 1804e1808. [5] A.T. Nientiedt, W. Jeitschko, Inorg. Chem. 37 (1998) 386e389. [6] S. Komatsuzaki, Y. Ohki, M. Sasaki, Y. Takahashi, K. Takase, Y. Takano, K. Sekizawa, AIP Conf. Proc. 850 (2006) 1255e1256. [7] H. Kabbour, L. Cario, F. Boucher, J. Mater. Chem. 15 (2005) 3525e3531.
[8] Y. Kamihara, H. Hiramatsu, M. Hirano, R. Kawamura, H. Yanagi, T. Kamiya, H. Hosono, J. Am. Chem. Soc. 128 (2006) 10012e10013. [9] S. Lebegue, Phys. Rev. B 75 (2007) 035110/1e035110/5. [10] AC Larson, RB Von Dreele, General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR 86-748, 2000. [11] SADABS: Absorption Correction, Bruker AXS Inc., Madison, USA, 2000. [12] G.M. Sheldrick, SHELXL: A Program for Crystal Structure Refinement, Universita¨t Go¨ttingen, Go¨ttingen, 1997. [13] Low Level Measurements, Precision DC Current, Voltage and Resistance Measurements, Keithley Instruments, Inc., Cleveland, Ohio, 1998. [14] D. Johrendt, C. Felser, O. Jepsen, O.K. Andersen, A. Mewis, J. Rouxel, J. Solid State Chem. 130 (1997) 254e265. [15] W.K. Hofmann, W. Jeitschko, J. Solid State Chem. 51 (1984) 152e158. [16] W. Jeitschko, U. Meisen, M.H. Mo¨ller, M. Reehuis, Z. Anorg. Allg. Chem. 527 (1985) 73e84. [17] R.N. Shelton, H.F. Braun, E. Musick, Solid State Commun. 52 (1984) 797e799. [18] M. Francois, G. Venturini, J.F. Mareche, B. Malaman, B. Roques, J. LessCommon Met. 113 (1985) 231e237. [19] W. Jeitschko, R. Glaum, L. Boonk, J. Solid State Chem. 69 (1987) 93e100. [20] M. Hirjak, P. Lejay, B. Chevalier, J. Etourneau, P. Hagenmuller, J. LessCommon Met. 105 (1985) 139e148. [21] R.J. Cava, H. Takagi, H.W. Zandbergen, J.J. Krajewski, W.F. Peck Jr., T. Siegrist, B. Batlogg, R.B. van Dover, R.J. Felder, et al., Nature (London) 367 (1994) 252e253. [22] A. Simon, Angew. Chem., Int. Ed. Engl. 36 (1997) 1789e1806. [23] We have synthesized LaFePO and found a significantly higher superconducting transition temperature (TC ¼ 6.5 K). The lower values given by Kamihara et al. [8] are presumably caused by contamination with ferromagnetic Fe2P. M. Tegel, Synthese, Struktur und Supraleitung quaterna¨rer Oxidphosphide, Ludwig-Maximilians-Universita¨t Mu¨nchen, Mu¨nchen, 2007. [24] Z. Otwinowski, W. Minor, Methods Enzymol. 276 (1997) 307e326. [25] T. Watanabe, H. Yanagi, T. Kamiya, Y. Kamihara, H. Hiramatsu, M. Hirano, H. Hosono, Inorg. Chem. 46 (2007) 7719e7721.
Solid State Sciences 10 (2008) 193e197 www.elsevier.com/locate/ssscie
Synthesis, crystal structure and superconductivity of LaNiPO Marcus Tegel, Daniel Bichler, Dirk Johrendt* Department Chemie und Biochemie der Ludwig-Maximilians-Universita¨t Mu¨nchen, Butenandtstrasse 5-13 (Haus D), 81377 Mu¨nchen, Germany Received 16 August 2007; received in revised form 22 August 2007; accepted 27 August 2007 Available online 2 September 2007
Abstract Single crystals of LaNiPO were synthesized by reacting La, P and NiO at 1173 K in a tin flux under argon atmosphere. A phase analysis and crystal structure determination was carried out using X-ray powder and single crystal methods. The quaternary phosphide oxide crystallizes in the tetragonal ZrCuSiAs structure (P4/nmm, a ¼ 404.53(1) pm, c ¼ 810.54(3) pm, Z ¼ 2), which is characterized by layers of edge-sharing 2 2 N ½La4=4 O and N ½NiP4=4 tetrahedra alternating along [001]. LaNiPO is a Pauli paramagnetic metal at room temperature and becomes a type-II superconductor at TC ¼ 4.3 K with an upper critical field Hc2 z 320 mT at 1.8 K. Connections between the crystal chemistry and the occurrence of superconductivity in LaNiPO and related ThCr2Si2 type compounds are discussed. Ó 2007 Elsevier Masson SAS. All rights reserved. Keywords: Phosphide oxides; Lanthanum; Nickel; Crystal structure; Superconductivity
1. Introduction Quaternary rare earth transition metal phosphide oxides with the tetragonal ZrCuSiAs structure [1] are known for more than ten years [2]. ZrCuSiAs was first described as a filled PbFCl type; silicon and copper occupy the tetrahedral holes and arsenic occupies the octahedral holes in an fcc packing of zirconium. But the two-dimensional order of the atoms in the tetrahedral holes leads to a more layer-like structure, thus also the related phosphide oxides RMPO (R ¼ rare earth metal, M ¼ transition metal) are better described as alternating layers of edge-sharing MP4/4 and R4/4O tetrahedra. Among the iron group compounds, the series RFePO (R ¼ LaeNd, Sm, Gd), RCoPO (R ¼ LaeNd, Sm) and RRuPO (R ¼ LaeNd, Sm, Gd) are known to date [2], as well as the manganese phosphide oxides RMnPO (R ¼ LaeNd, Sm, GdeDy) [3]. Recently, dimorphic CeZnPO and PrZnPO were reported [4], forming either the tetragonal ZrCuSiAs or the rhombohedral NdZnPO type, depending on the synthesis conditions [5].
Even though numerous RMPO compounds have been synthesized and characterized, virtually nothing is known about their electronic and magnetic properties. Magnetic data are only reported for the antimonide oxides CeZnSbO and PrZnSbO [6]. But beyond the typically localized magnetism of the trivalent rare earth ions, especially phosphide oxides with open-shell 3d-metals may display interesting magnetic and electronic phenomena. First motivating results are the antiferromagnetic order at room temperature found in the ZrCuSiAs type phosphide fluoride BaMnPF [7] and the recently reported superconductivity of LaFePO [8,9]. We have started systematic investigations about the structureeproperty relationships of quaternary phosphide oxides with 3d-transition metals with particular attention to magnetic and electronic phenomena. In the present paper, we report on the synthesis, crystal structure and superconductivity of the first nickel phosphide oxide LaNiPO. 2. Experimental 2.1. Synthesis
* Corresponding author. Tel.: þ49 (0)89 2180 77430; fax: þ49 (0)89 2180 77431. E-mail address: [email protected] (D. Johrendt). 1293-2558/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2007.08.016
Single crystals of LaNiPO were prepared by heating a mixture of 375.0 mg La (99.9%, Smart Elements), 201.7 mg
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M. Tegel et al. / Solid State Sciences 10 (2008) 193e197
NiO (99.99%, Sigma-Aldrich) and 83.6 mg P (red, 99þ%, Sigma-Aldrich) with 2000 mg Sn (99.99%, Alfa Aesar) in an alumina crucible, which was sealed in a silica tube under an atmosphere of purified argon. The sample was heated to 1173 K at a rate of 40 K/h, kept at this temperature for 10 days and slowly cooled down to room temperature at a rate of 3 K/h. The crucible was smashed and the tin bar dissolved in 6 M HCl at room temperature. The remaining sample consisted of single crystals of LaNiPO beside small amounts of LaNi2P2 (7%), Ni2SnP (4%) and Ni3Sn4 (<1%). Further attempts to optimize the synthesis conditions with regard to reaction temperature or duration were unsuccessful. Samples prepared directly from the starting material without tin flux yielded only small amounts of LaNiPO besides LaNi2P2 as main product. 2.2. Characterization X-ray powder patterns of finely ground samples were recorded on a Stoe Stadi-P diffractometer (Mo-Ka1 radiation, Ge(111)-monochromator, l ¼ 70.93 pm, 7 PSD-detector, silicon as external standard) at room temperature. The powder data was analyzed by the Rietveld method using the GSAS suite [10]. Single crystal data were collected in oscillation mode using an Enraf-Nonius k-CCD, equipped with a rotating anode (Mo-Ka radiation, graphite-monochromator, l ¼ 71.073 pm). The intensities were corrected for absorption with SADABS [11] and refined against F2 using SHELXL97 [12] with anisotropic displacement parameters for all atoms. Refinements of the site occupation factors converged to full occupation of all sites within standard deviations. Starting parameters were taken from isostructural PrFePO [2]. 2.3. Magnetic measurements Magnetization measurements were performed using a Quantum-Design SQUID magnetometer (MPMS-XL5) between 1.8 and 300 K with magnetic flux densities up to 5 T. A cold pressed pellet of LaNiPO was put into a gelatin capsule and fixed in a straw as sample holder. Zero-field-cooling (Shielding) and field-cooling (Meissner) measurement cycles were performed at 1 mT between 1.8 and 8 K in DC mode. The magnetic susceptibility between 2 and 300 K was measured at 1 T. The magnetization isotherm at 8 K is linear up to 5 T and reveals only small traces of ferromagnetic contamination. 2.4. Electric resistivity measurements The electric resistivity of a LaNiPO sample was measured using a DC 4-point current reversal method [13]. A cold pressed pellet was mounted onto a special sample holder with Apiezon-N grease, contacted to four copper wires with silver conducting paint and inserted into the cryogenic system of the SQUID magnetometer. The voltage drop along the sample at a constant current of 10 mA was measured with a Keithley nanovoltmeter between 1.8 and 8 K in steps of 0.2 K.
3. Results The structure determination of a LaNiPO single crystal confirmed the tetragonal ZrCuSiAs structure (space group P4/nmm), which is isotypic to LaFePO and other related phosphide oxides. The new compound represents the first nickel phosphide oxide of this structural family. Crystallographic data and selected bond lengths and bond angles are listed in Tables 1 and 2. Further details about the crystal structure determination may be obtained from: Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany, e-mail: [email protected], on quoting the registration No. CSD-391427. In order to detect and quantify significant impurity phases, we refined the X-ray powder pattern of a sample by the Rietveld method using the single crystal data of LaNiPO as starting parameters. The experimental pattern as shown in Fig. 1 was fitted using three phases, namely 89.2% LaNiPO, 6.9% LaNi2P2 and 3.9% Ni2SnP. Additional small traces (<1%) of Ni3Sn4 were also detected, but not included in the Rietveld fit. Thus, the synthesis yielded about 90% LaNiPO and the closely related LaNi2P2 as the main byproduct. The crystal structure of LaNiPO consists of layers of edgesharing La4/4O and NiP4/4 tetrahedra alternating along [001] as depicted in Fig. 2. The NieP distance is 226.5(2) pm, i.e. Table 1 LaNiPO: crystallographic data and details of the structure refinement Crystal system Space group Lattice constant a (pm) Lattice constant c (pm) Unit cell determination method Formula units (Z ) Calculated density Dx (g/cm3) Molar volume Vm (cm3/mol) Crystal dimensions (mm3) Diffractometer and wavelength hmax ¼ kmax lmax qmax ( ) F(000) Absorption coefficient m (mm1) Data reduction and absorption correction Number of refined parameters Reflections (unique) Rint/Rs Reflections with jIj 2s(I ) Structure determination and refinement Scattering factors R1/R1 with jIj 2s(I ) wR2/goodness of fit (GooF) Residual density, max (e 106 pm3) Residual density, min (e 106 pm3)
Tetragonal P4/nmm (no. 129, origin choice 2) 404.53(1) 810.54(3) Rietveld fit 2 6.124 39.94 0.05 0.05 0.02 Enraf-Nonius k-CCD, Mo-Ka: l ¼ 71.073 pm 5 10 27.29 216 23.43 SCALEPACK [24], SADABS [11] 11 2366 (117) 0.0456/0.0220 114 Empirical/SHELXL-97 [12] International Tables, vol. C 0.0274/0.0269 0.0713/1.272 2.572 0.802
M. Tegel et al. / Solid State Sciences 10 (2008) 193e197 Table 2 Atomic parameters, anisotropic displacement factors and selected bond lengths (pm) and angles ( ) for LaNiPO (Uij in pm2; U11 ¼ U22; U12 ¼ U13 ¼ U23 ¼ 0) Atom
Wyck.
x/a
y/b
z/c
U11
U33
Ueq
La Ni P O
(2c) (2b) (2c) (2a)
1/4 3/4 1/4 3/4
1/4 1/4 1/4 1/4
0.15190(9) 1/2 0.6257(5) 0
87(4) 179(7) 134(13) 100(30)
116(5) 147(8) 164(17) 100(40)
97(4) 168(5) 144(9) 101(19)
LaeO LaeP LaeNi NieP NieNi
236.79(4) 338.1(2) 347.2(1) 226.5(2) 286.05(1)
(4) (4) (4) (4) (4)
LaeOeLa LaeOeLa PeNieP PeNieP
105.69(1) 117.34(3) 101.7(1) 126.5(2)
close to the sum of the covalent radii of 225 pm. As it can be recognized by PeNieP angles as large as 126.5(2) , the NiP4 tetrahedra are significantly compressed along [001]. This holds true to a smaller extent for the OLa4 tetrahedra with LaeOeLa angles of 117.3(1) and a LaeO distance of 236.8(1) pm. Lanthanum is eightfold coordinated by four oxygen and four phosphorus atoms (LaeP 338.1(2) pm), forming a distorted tetragonal antiprism. The coordination sphere of nickel is completed by four neighboring nickel atoms within the same layer at a distance of 286.1(1) pm, this can be considered as weak NieNi bonds at best. A remarkably small c/a ratio of LaNiPO (2.00) in comparison with LaFePO (2.15) and LaCoPO (2.11) results from the compression of the tetrahedra mentioned above. We believe this compression is a consequence of a weaker metalemetal bonding between the nickel atoms within the phosphide layers. The scenario is very similar among the closely related ThCr2Si2 type compounds, where metalemetal bonding plays
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an important role. As we have shown earlier [14], an increasing electron count along the series RM2P2 (R ¼ rare earth metal, M ¼ Fe, Co, Ni) fills antibonding bands of strongly overlapping 3dxy orbitals and weakens the metalemetal bonds within the phosphide layer. As the MeM bonds become weaker and longer, the layer is expanded in the (ab)-plane and compressed perpendicular (along [001]) in order to avoid longer MeP bonds. In the case of the ThCr2Si2 type phosphides, this mechanism decreases the distance between the MP4/4 layers and can lead to the formation of short PeP bonds between adjacent layers. This can be observed in LaNi2P2 [15], which has short PeP bonds (239 pm), whereas no such bonds are formed in LaCo2P2 and LaFe2P2 [16], because the latter two have fewer antibonding electrons and therefore stronger metalemetal bonds. In LaNiPO, the NiP4/4 sheets are separated by OLa4/4 layers and therefore direct PeP bonds between the layers are impossible. On the other hand, the electron count within the NiP4/4 layer of LaNiPO is smaller than the electron count in LaNi2P2. Assuming La3þ in both cases, only one electron is transferred to the phosphide layer in LaNiPO according to (LaO)þ(NiP) but 1.5 electrons are transferred in La3þ(Ni2P2)3. Therefore the occupation of the NieNi antibonding bands is expected to be smaller in LaNiPO than in LaNi2P2 and the phosphide layer is less destabilized in the phosphide oxide. First magnetic measurements of a LaNiPO sample showed weak paramagnetism (cmol z 5 104 cm3/mol), varying only slightly with temperature, as it is common for metallic materials. Magnetization isotherms showed linear behavior but indicated small traces of an unknown ferromagnetic contamination equivalent to w10 ppm nickel metal; therefore we are unable to quantify the Pauli paramagnetism.
Fig. 1. X-ray powder pattern (þ) and Rietveld fit () of the LaNiPO sample. Refined phases are LaNiPO (89.2%), LaNi2P2 (6.9%) and Ni2SnP (3.9%). The residual intensities are caused by traces of Ni3Sn4, which was not included in the Rietveld fit.
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M. Tegel et al. / Solid State Sciences 10 (2008) 193e197
Fig. 2. Crystal structure of LaNiPO.
The sample, which was cooled down to 1.8 K in a zero magnetic field (zfc), showed very strong diamagnetism as it is typical of superconductors. Subsequent shielding (zfc) and Meissner (fc) measurements confirmed the superconducting transition of LaNiPO at TC ¼ 4.3 K (Fig. 3). The volume susceptibility of the sample reaches about 70% of the ideal diamagnetic shielding (c ¼ 1) at 1.8 K, therefore it is obvious that the superconductivity is not caused by an impurity phase. We also performed preliminary measurements of the critical flux densities. LaNiPO is a type-II superconductor with small critical flux densities of Bc1 z 3 mT and Bc2 z 320 mT and a calculated thermodynamic critical flux density Bc,th z 32 mT at 1.8 K. Fig. 3 also shows the electrical resistance of the sample between 1.8 and 8 K. Due to the rather arbitrary fixed contacts and an irregular shaped sample, we are unable to give specific resistivity values, but the resistance was about 100 mU at 8 K. Nevertheless, the superconducting transition at 4.3 K is clearly discernible and the resistance drops between 4.3 and 1.8 K to 3% of the value at 8 K. The critical temperature found here is in excellent agreement with the magnetization measurements (dotted line in Fig. 3). The rather broad transition (within w2 K) may be a hint at granular superconductivity or inhomogeneities of the sample. 4. Discussion LaNiPO is the first nickel phosphide oxide and the second superconducting compound in this family. Thus, the superconductivity of LaFePO does not seem to be a unique observation in this class of compounds and it is probable that superconductivity will occur frequently here. We especially want to indicate the relationship between LaNiPO and the ThCr2Si2 type compounds, where superconductivity is extremely rare with respect to the large number of about 700 compounds of this type [17]. Superconductivity has only been observed in
Fig. 3. Magnetic susceptibility (B ¼ 1 mT) and electrical resistance (I ¼ 10 mA) of the LaNiPO sample between 1.8 and 8 K.
LaIr2Ge2 (TC ¼ 1.5 K) [18], LaRu2P2 (TC ¼ 4.1 K) [19] and YIr2xSi2þx (TC ¼ 2.8e3.05 K) [20]. Contrary to this, the ThCr2Si2 type related boride-carbides RNi2B2C (R ¼ Y, Tm, Er, Ho, Lu) are well-known superconductors with critical temperatures up to 16 K for R ¼ Lu [21]. We have discussed earlier [14] that superconductivity in ThCr2Si2 type and related compounds like LuNi2B2C is associated with interactions between the transition metal atoms, which generate a peak in the density-of-states (DOS) close to the Fermi level. Very recently it was shown that superconducting LaFePO shows the same property in its electronic structure [9]. We assume that the superconductivity of LaRu2P2, LuNi2B2C, LaFePO and LaNiPO originates in the transition metal layers (RuP, NiB, FeP and NiP), which provide the necessary, but not sufficient electronic conditions according to the flat band/steep band scenario proposed by Simon [22]. We also believe that the separating layers play an important role for superconductivity. Fig. 4 shows the structures of LaRu2P2, LuNi2B2C and LaNiPO in comparison. In ThCr2Si2 type compounds like LaRu2P2 or LaNi2P2, where RuP or NiP layers are separated by lanthanum atoms, superconductivity occurs rarely and at low temperatures. But if the analogous NiB layers are separated by LuC, as in LuNi2B2C, superconductivity with relatively high TC
M. Tegel et al. / Solid State Sciences 10 (2008) 193e197
197
Fig. 4. Crystal structures of the superconductors LaRu2P2 (TC ¼ 4.1 K), LuNi2B2C (TC ¼ 16 K) and LaNiPO (TC ¼ 4.5 K).
emerges. Accordingly, LaNi2P2 and LaFe2P2 are normal metals, but if the NiP or FeP layers are sandwiched between LaO layers, as it is the case for LaNiPO and LaFePO, both become superconducting at TC ¼ 4.3 and 6.5 K, respectively [23]. These ideas open up interesting prospects of synthesizing further transition metal phosphide oxides with different separating layers in order to study the superconductivity in this class of materials in more detail. During the reviewing process, we took notice of a recently published article on LaNiPO [25]. Therein, a superconducting transition temperature of w3 K is reported in contrast to our data showing TC ¼ 4.3 K. The superconducting volume fraction of 38% (cg ¼ 5.3 103 emu/g at 1.8 K) is considerably smaller than that of our sample, which shows 73% superconducting phase (cg ¼ 9.5 102 emu/g at 1.8 K). We believe that the reasons for these differences are small amounts of ferromagnetic contaminations of the samples, which we also suspect in the case of LaFePO [23]. References [1] V. Johnson, W. Jeitschko, J. Solid State Chem. 11 (1974) 161e166. [2] B.I. Zimmer, W. Jeitschko, J.H. Albering, R. Glaum, M. Reehuis, J. Alloys Compd. 229 (1995) 238e242. [3] A.T. Nientiedt, W. Jeitschko, P.G. Pollmeier, M. Brylak, Z. Naturforsch., B: Chem. Sci. 52 (1997) 560e564. [4] H. Lincke, T. Nilges, R. Po¨ttgen, Z. Anorg. Allg. Chem. 632 (2006) 1804e1808. [5] A.T. Nientiedt, W. Jeitschko, Inorg. Chem. 37 (1998) 386e389. [6] S. Komatsuzaki, Y. Ohki, M. Sasaki, Y. Takahashi, K. Takase, Y. Takano, K. Sekizawa, AIP Conf. Proc. 850 (2006) 1255e1256. [7] H. Kabbour, L. Cario, F. Boucher, J. Mater. Chem. 15 (2005) 3525e3531.
[8] Y. Kamihara, H. Hiramatsu, M. Hirano, R. Kawamura, H. Yanagi, T. Kamiya, H. Hosono, J. Am. Chem. Soc. 128 (2006) 10012e10013. [9] S. Lebegue, Phys. Rev. B 75 (2007) 035110/1e035110/5. [10] AC Larson, RB Von Dreele, General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR 86-748, 2000. [11] SADABS: Absorption Correction, Bruker AXS Inc., Madison, USA, 2000. [12] G.M. Sheldrick, SHELXL: A Program for Crystal Structure Refinement, Universita¨t Go¨ttingen, Go¨ttingen, 1997. [13] Low Level Measurements, Precision DC Current, Voltage and Resistance Measurements, Keithley Instruments, Inc., Cleveland, Ohio, 1998. [14] D. Johrendt, C. Felser, O. Jepsen, O.K. Andersen, A. Mewis, J. Rouxel, J. Solid State Chem. 130 (1997) 254e265. [15] W.K. Hofmann, W. Jeitschko, J. Solid State Chem. 51 (1984) 152e158. [16] W. Jeitschko, U. Meisen, M.H. Mo¨ller, M. Reehuis, Z. Anorg. Allg. Chem. 527 (1985) 73e84. [17] R.N. Shelton, H.F. Braun, E. Musick, Solid State Commun. 52 (1984) 797e799. [18] M. Francois, G. Venturini, J.F. Mareche, B. Malaman, B. Roques, J. LessCommon Met. 113 (1985) 231e237. [19] W. Jeitschko, R. Glaum, L. Boonk, J. Solid State Chem. 69 (1987) 93e100. [20] M. Hirjak, P. Lejay, B. Chevalier, J. Etourneau, P. Hagenmuller, J. LessCommon Met. 105 (1985) 139e148. [21] R.J. Cava, H. Takagi, H.W. Zandbergen, J.J. Krajewski, W.F. Peck Jr., T. Siegrist, B. Batlogg, R.B. van Dover, R.J. Felder, et al., Nature (London) 367 (1994) 252e253. [22] A. Simon, Angew. Chem., Int. Ed. Engl. 36 (1997) 1789e1806. [23] We have synthesized LaFePO and found a significantly higher superconducting transition temperature (TC ¼ 6.5 K). The lower values given by Kamihara et al. [8] are presumably caused by contamination with ferromagnetic Fe2P. M. Tegel, Synthese, Struktur und Supraleitung quaterna¨rer Oxidphosphide, Ludwig-Maximilians-Universita¨t Mu¨nchen, Mu¨nchen, 2007. [24] Z. Otwinowski, W. Minor, Methods Enzymol. 276 (1997) 307e326. [25] T. Watanabe, H. Yanagi, T. Kamiya, Y. Kamihara, H. Hiramatsu, M. Hirano, H. Hosono, Inorg. Chem. 46 (2007) 7719e7721.