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Journal of Alloys and Compounds 831 (2020) 154697

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Journal of Alloys and Compounds

journal homepage: http: / /www.elsevier .com/locate/ ja lcom

High-pressure synthesis, structure and properties of new ternarypnictides La3TiX5 (X ¼ P, As)

Lei Duan a, b, 1, Jun Zhang a, b, 1, Xiancheng Wang a, *, Jianfa Zhao a, b, Lipeng Cao a, b,Wenmin Li a, b, Zheng Deng a, Runze Yu a, Zhi Li c, **, Changqing Jin a, b, d, ***

a Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, Chinab School of Physics, University of Chinese Academy of Sciences, Beijing, 100190, Chinac College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, 210094, Chinad Materials Research Lab at Songshan Lake, Dongguan, 523808, China

a r t i c l e i n f o

Article history:Received 16 December 2019Received in revised form6 March 2020Accepted 7 March 2020Available online xxx

Keywords:One dimensional crystal structureMetalHigh pressureElectronic band structure

* Corresponding author.** Corresponding author.*** Corresponding author. Beijing National LaboraPhysics, Institute of Physics, Chinese Academy of Scie

E-mail addresses: wangxiancheng@iphy.ac.cn (X(Z. Li), jin@iphy.ac.cn (C. Jin).

1 These authors contributed equally.

https://doi.org/10.1016/j.jallcom.2020.1546970925-8388/© 2020 Elsevier B.V. All rights reserved.

a b s t r a c t

The new ternary pnictides La3TiX5 (X ¼ P, As) and the solid solutions of La3Ti(Sb1-xAsx)5 (x ¼ 0e1) andLa3Ti(As1-yPy)5 (y ¼ 0e1) were synthesized under high pressure and high temperature conditions. Thecrystal structure of La3TiX5 (X ¼ P, As) was determined from the X-ray powder diffraction data. Thesecompounds crystallize in a hexagonal Hf5Sn3Cu-anti type structure with the space group P63/mcm andlattice parameters a ¼ 8.7927 Å and c ¼ 5.7643 Å for La3TiP5, and a ¼ 9.0267 Å and c ¼ 5.9071 Å forLa3TiAs5, in which the face-sharing TiX6 octahedral chains running along the c axis are separated by La3þ

in the interstice sites with a distance about 9 Å, presenting a strong one-dimensional crystal structure.Physical properties measurements indicate La3TiX5 (X ¼ P, As) are Pauli paramagnetic metals. The cal-culations reveal that La3þ ions are not perfectly ionic owing to the non-negligible contributions of La tothe density of state (DOS) at the Fermi level and bridge the X atoms between the chains, which make thesamples to be 3-dimensional metals. What’s more, the change of X in La3TiX5 from Sb to P increases theionicity of La3þ and thus decreases the electron hopping between the conducting chains.

© 2020 Elsevier B.V. All rights reserved.

1. Introduction

Ternary rare-earth 3d-transition metal pnictides RE-M-X (X ¼ P,As, Sb, Bi) give rise to a vast class of materials adopting a plethora ofcrystal structure types and displaying a wide variety of physicalproperties [1e7]. The systems containing a late 3d-transition metalare now relatively well known, whereas those containing an early3d-transition metal remain poorly investigated. When M is V or Cr,a few ternary compounds are known, such as REMSb3 (RE¼ LaeNd,Sm, Gd, Dy, Yb) and EuCr2As2 [7e9]. In the RE-Ti-Sb systems, sys-tematic investigations have been reported and the existence of fourseries of ternary phases is described: RE3TiSb5 (RE ¼ LaeNd, Sm)

tory for Condensed Matternces, Beijing, 100190, China.. Wang), zhili@njust.edu.cn

[6,10], RETi3Sb4 (RE ¼ Ce, Nd, Sm) [1,11], RE2Ti7Sb12 (RE ¼ La-Nd)[11] and RE2Ti11-xSb14þx (RE ¼ Sm, Gd, Tb, Yb) [12]. These ternarycompounds display complex crystal chemistry including TieTi andSbeSb bonding interaction. In particular, RE3TiSb5 (RE ¼ LaeNd,Sm), crystallizing into hexagonal Hf5Sn3Cu-anti type structure,contains face-sharing TiSb6 octahedral chains and hypervalent Sb-chains aligned along the c axis and separated by RE3þ and thus,displays a strong one-dimensional crystal structure. It has beenreported that their magnetic properties result from the localized f-electrons of rare-earth metal [13]. For the RE-Ti-Bi systems(RE ¼ rare-earth metal), a few ternary compounds are reported inthe literature, such as RETi3Bi4 (RE ¼ La, Ce, Eu) [1,14], Eu3Ti8Bi10[15] and RE3TiBi5 (RE ¼ La, Ce) [1,4]. However, in contrast to thesecompounds with heavier pnicogen, no ternary phase in the RE-Ti-X(X ¼ P, As) systems has been reported up to now.

Besides temperature and composition, pressure is another keyparameter to contribute increasingly to innovations in materialssciences. The prominent effects of pressure are to shorten atomicdistance, to enhance charge transfer, and to initiate chemical re-actions forbidden at ambient pressure. High pressure is capable to

L. Duan et al. / Journal of Alloys and Compounds 831 (2020) 1546972

generate plenty of new materials or new phases, which can hardlybe synthesized under ambient pressure. Motivated in part by thelack of information on titanium pnictides, we set out to conduct anexploratory work in the LaeTi-X (X ¼ P, As) systems using highpressure technique. Here, we report on the synthesis, crystal andelectronic structure, and physical properties of the new ternarycompounds of La3TiAs5 and La3TiP5, as well as the solid solutions ofLa3Ti(Sb1-xAsx)5 (x ¼ 0e1) and La3Ti(As1-yPy)5 (y ¼ 0e1).

2. Experimental

Commercially available lumps of La (Alfa,>99.999% pure), lumpsof As (Alfa, >99.999% pure), Sb powder (Alfa, >99.99% pure), Ppowder (Alfa, >99% pure) and Ti powder (Alfa, >99.99% pure) wereused as the starting materials. The precursor LaX (X ¼ Sb, As and P)were prepared by the reaction of La lumps and X powders at 700 �Cin an evacuated quartz tube. We tried to synthesize La3TiX5 (X ¼ P,As) at ambient pressure with the detail similar to that for La3TiSb5[10]. However, we cannot obtain the samples of La3TiX5 (X ¼ P, As)via ambient synthesis. Here, the La3Ti(Sb1-xAsx)5 (x ¼ 0e1) andLa3Ti(As1-yPy)5 (y ¼ 0e1) samples were synthesized successfullyunder high pressure and high temperature conditions. The LaX, Tiand X powders were mixed according to the elementary ratio ofstoichiometric La3TiX5, pressed into a pellet with a diameter of6 mm, and then subjected to high-pressure synthesis under 5.5 GPapressure and 1000 �C for 40 min in a cubic-anvil-type high-pres-sure apparatus, of which the details have been reported in ref (16,17) [16,17].

The X-ray diffraction (XRD) was conducted on a Rigaku UltimaVI (3 KW) diffractometer using Cu Ka radiation generated at 40 kVand 40 mA. The Rietveld refinements on the diffraction patternswere performed using GSAS software packages [18]. The dc mag-netic susceptibility measurement was carried out using a super-conducting quantum interference device (SQUID). The resistancewas measured by four-probe electrical conductivity methods inphysical property measuring system (PPMS).

The first-principles calculations based on density functionaltheory implemented in VASP were carried out within a primitivecell with an 8 � 8 � 16 k-point grid [19]. The projector augmentedwave pseudopotentials with Perdew, Burke, and Ernzerhof (PBE)exchange-correlation and 450 eV energy cutoff were used in ourcalculation [20,21]. The experimental lattice parameters from XRDwere adopted.

3. Results and discussion

3.1. Crystal structure

Polycrystalline samples La3TiP5 and La3TiAs5 were preparedunder high-temperature and high-pressure conditions. Fig. 1(a andb) displays the refinements for the XRDs of La3TiP5 and La3TiAs5,respectively. Here, the structure of La3TiSb5 with the space group ofP63/mcm (193) was adopted as the initial model for the Rietveldrefinements [10], which smoothly converged to c2 ¼ 4.1, Rp ¼ 3.2%and Rwp¼ 5.2% for La3TiP5 and c2¼ 4.5, Rp¼ 3.7% and Rwp¼ 5.6% forLa3TiAs5. The summary of the crystallographic data is listed inTable 1, and some selected important distances and angles are lis-ted in Table 2. For comparing, the selected distances and angles ofisostructural La3TiSb5 and La3TiBi5 are listed in Table 2 as well. Infact, we have tried the synthesis at ambient pressure via sealing theprecursors of LaAs(P), As(P) and Ti powders in an evacuated quartztube and sintering at different temperatures (800 �C and 1000 �C).However, we didn’t obtain the samples of La3TiAs(P)5 at ambientpressure. Since the quartz tube were intact during the ambientpressure sintering process, it is speculated that the reason of failing

to obtain La3TiAs(P)5 at ambient pressure is complex and not justthe high vapor pressure of P and As.

Fig. 2(a) presents the sketch of crystal structure of La3TiX5viewed with the projection along the [001] direction. The structureconsists of face-sharing TiX6 octahedral chains along the c axis,which are arranged in a triangular lattice form. The X1 anions,occupying the site of (x, 0, 1/4), surround the center ions of Ti toform TiX6 octahedrons. Besides the TiX6 chains, the anions X2located at the center of the triangular lattice with the site of (1/3, 2/3, 0) are space-equally aligned along the c axis to form the X-chains.The corresponding intrachain distance of X2-X2, is 2.8819 Å forLa3TiP5 and 2.9537 Å for La3TiAs5, which is a little larger than thetypical bond length of PeP (2.2e2.3 Å) [22] and AseAs (~2.5 Å) [23],respectively. The moderate intrachain distance of X2-X2 suggeststhe X2 anions in the linear X-chains are hypervalent with a formalcharge of X22�. The similar hypervalent Bi2� has been reported inLa3TiBi5 compound [4]. As for the interchain distances of X2-X2 andX1-X2 shown in Table 2, they are so large enough that there is nodirectly orbital overlap between each two chains of the X-chainsand TiX6 octahedral chains. Fig. 2(b) shows the partial structure forLa3TiX5, displaying the connection of TiX6 chains by face-sharingLaX9 polyhedrons. There are nine X-ligands surrounding the cen-ter La3þ ion, of which four X1 ligands come from the same TiX6

chain, one X1 ligands from the other TiX6 chain and the other fourX2 ligands come from the X-chains. The distances of LaeP in theLaP9 polyhedron in La3TiP5 range from 2.9194 Å to 3.1228 Å, and thebond lengths between the nine As-ligands and the center La3þ ionsin La3TiAs5 range from 3.0088 Å to 3.1878 Å. In addition, it is notedthat the corresponding distance of LaeLa in La3TiAs5 and La3TiP5 is3.6726 Å and 3.5864 Å, respectively, which are comparablewith theinteratomic distance in La metal (~3.65 Å) [24].

To further study the evolution of crystallographic data includinglattice, some important distances and angles from La3TiSb5 toLa3TiP5, we successfully synthesized the solid solutions of La3T-i(Sb1-xAsx)5 (x ¼ 0e1) and La3Ti(As1-yPy)5 (y ¼ 0e1) under highpressure and high temperature conditions. Fig. 3 (a-b, d-e) displaysthe x-ray diffraction patterns of the solid solutions. Whenincreasing the doping level of x and y, the peaks shift monotonouslytowards a high-angle direction, which demonstrates that the Asand P atoms are doped into La3TiSb5 and La3TiAs5, respectively. Wealso carried out the refinements for the X-ray diffraction data of thesolid solutions of La3Ti(Sb1-xAsx)5 (x ¼ 0e1) and La3Ti(As1-yPy)5(y ¼ 0e1) (see Figs. S1eS9 and Tables S1eS9). The lattice param-eters a and c can be obtained from the refinements. The dopingdependence of lattice parameters is plotted as shown in Fig. 3(c, f).The obtained lattice constants a¼ 9.5265 (7) Å and c¼ 6.2770 (5) Åof La3TiSb5 agree with those reported in previous work [10]. Theselattice constants decrease linearly as the doping level x and y in-crease. From La3TiSb5 to La3TiAs5 and to La3TiP5, the unit cell vol-ume is shrunk by 15.6% and 21.9%, respectively.

Fig. 4 presents the doping dependence of the bond angles a ofX1-Ti-X1 in the TiX6 octahedrons (shown in Fig. 2(b)) and the dis-tances of TieTi in TiX6 octahedral chains. The results reveal that allthe TiX6 octahedrons are slightly compressed along the c axis.When X varies from large Sb atoms to smaller P atoms, the dis-tances of TieTi decreases from 3.1385 Å to 2.8822 Å. For La3TiX5, theTieTi distances are comparable with that in an elemental titaniummetal (~2.87e2.90 Å), which suggests that there exist bonding in-teractions between corresponding Ti ions. In addition, the elec-trostatic repulsion between Ti ions in the center of correspondingface-sharing TiX6 octahedrons becomes stronger as the TieTi dis-tance decreases, which tends to elongate the octahedrons. Thereby,when X varies from Sb to P, the compression of TiX6 octahedrons ispartially released and the bond angles a increases from 87.329� to88.697�, as shown in Fig. 4.

Fig. 1. The X-ray diffraction patterns and the refinements with a hexagonal Hf5Sn3Cu-anti type structure and the space group of P63/mcm for (a) La3TiP5 and (b) La3TiAs5,respectively.

Table 1The summary of crystallographic data at room-temperature for La3TiP5 and La3TiAs5, respectively.

Hexagonal structure; Space group: P63/mcm, No.193

La3TiP5 La3TiAs5a ¼ 8.7927 (4) (Å) a ¼ 9.0265 (7) (Å)c ¼ 5.7643 (5) (Å) c ¼ 5.9070 (6) (Å)c2 ¼ 4.1; Rp ¼ 3.2% c2 ¼ 4.5; Rp ¼ 3.7%Rwp ¼ 5.2% Rwp ¼ 5.6%

Site Wyck. x y z 100 � U (Å)

La3TiP5La 6g 0.6213 (0) 0 1/4 1.05 (9)Ti 2b 0 0 0 1.44 (0)P1 6g 0.2400 (4) 0 1/4 2.18 (6)P2 4d 1/3 2/3 0 3.14 (2)La3TiAs5La 6g 0.6216 (5) 0 1/4 1.55 (6)Ti 2b 0 0 0 1.49 (4)As1 6g 0.2471 (9) 0 1/4 1.77 (1)As2 4d 1/3 2/3 0 2.71 (1)

Table 2Selected distances (Å) and angles (�) between adjacent atoms. * denotes the intra-chain distance and # denotes the interchain distance.(a) The data is from ref [4].

La3TiX5 La3TiP5 La3TiAs5 La3TiSb5 (a)La3TiBi5

*TieTi ( � 2) 2.8822 2.9535 3.1385 3.2322*X2-X2 ( � 2) 2.8822 2.9535 3.1385 3.2322#X2-X2 ( � 3) 5.0766 5.2116 5.5003 5.5996X1-X2 ( � 6) 3.7073 3.7594 3.9605 4.0253La-X1 ( � 2) 3.1224 3.1781 3.3723 3.4673La-X1 ( � 2) 2.9179 3.0051 3.1856 3.2645La-X1 ( � 1) 3.3352 3.3705 3.5244 3.5421La-X2 ( � 4) 3.1076 3.1902 3.3672 3.4291LaeLa ( � 2) 3.5857 3.6811 3.8856 3.9596Ti-X1 ( � 6) 2.5557 2.6841 2.8543 2.9373X1-Ti-X1(a) 88.679 87.531 87.329 87.255X1-Ti-X1(b) 91.321 92.469 92.671 92.745

L. Duan et al. / Journal of Alloys and Compounds 831 (2020) 154697 3

3.2. Physical properties

The resistivity dependent on temperature for La3TiX5 samplesare presented in Fig. 5(a). All the resistivity curves exhibit a metallicbehavior. For La3TiSb5, the room temperature resistivity rr is about

0.1 mU-cm, which is comparable with the previous work [4]. Thevalue of rr increases more than fifty times to 5.7 mU-cmwhen Sb isreplaced by P. The replacement of Sb with higher electronegativityof P should increase the ionic component of La3þ and reduce theinterchain electronic hopping, which will be discussed in followingcalculations, and thereby increase the resistivity.

Fig. 5(b and c) displays the temperature dependence of mag-netic susceptibility c of La3TiX5 samples measured at 1 T. Thesusceptibility curves demonstrate paramagnetic like behavior. Byfitting the susceptibility within the whole measured temperatureregion using modified Curie-Weiss law c ¼ c0 þ C/(T-Tq), where c0is the susceptibility independent of temperature, C is the Curieconstant and Tq is the Weiss temperature, we can obtain the Curieconstant and estimate the effective moment meff, which are 0.60 mBand 0.47 mB per molecular formula for La3TiP5 and La3TiAs5,respectively. The small value of meff suggests that the paramagneticlike behavior should arise from paramagnetic impurities. To sub-tract the contribution from impurities, we fit the susceptibility oflow temperature regionwith Curie-Weiss law. After subtracting theimpurity contribution, we obtain the corrected susceptibility~3.5 � 10�5 emu/mol for La3TiP5 and ~5.3 � 10�5 emu/mol forLa3TiAs5, which are almost temperature independent in the

Fig. 2. (a) The crystal structure of La3TiX5 with the projection along c axis, showing the triangular lattice form and chain structure characteristic. (b) The partial structure for La3TiX5,showing the bridge of LaX9 polyhedral between TiX6 octahedral chains and X-chains.

Fig. 3. (a) X-ray powder diffraction patterns of La3Ti(Sb1-xAsx)5 (x ¼ 0e1); (b) the enlarged view, exhibiting the peak shift; (c) the lattice parameter a and c versus doping level x; (d-f) the X-ray powder diffraction patterns, enlarged view and lattice parameter vs doping level y for La3Ti(As1-yPy)5 (y ¼ 0e1), respectively.

L. Duan et al. / Journal of Alloys and Compounds 831 (2020) 1546974

measured temperature region, as shown in Fig. 5(b and c). Thus,La3TiP5 and La3TiAs5 should be Paul paramagnetic metals, whichagrees with the work for the isostructural La3TiSb5 and will be

further confirmed by our following calculations about the localmoments for the system.

Fig. 4. The angles of X-Ti-X (a) in TiX6 octahedrons and distances of TieTi in TiX6

octahedral chains.

Fig. 5. (a) The resistivity dependent on temperature for La3TiX5 (X ¼ P, As, Sb). (b) and (c) TheLa3TiAs5, respectively. The red dashed line is the fit for the curve c vs temperature at low tethe references to colour in this figure legend, the reader is referred to the Web version of

L. Duan et al. / Journal of Alloys and Compounds 831 (2020) 154697 5

3.3. Calculations

We have performed the first-principles calculations to obtainmore insight into the physical properties from the band structureand partial density of state (PDOS) for La3TiP5 and La3TiAs5, asshown in Fig. 6(a) and (b), respectively. These band structures aresimilar to that of La3TiSb5 [4]. There are several bands crossing theFermi level along the c� axis including the G-A path, HeK path andL-M path, which suggests it is conducting along the chain direction.In addition, there are three bands that cut the Fermi level in the K-G-M plane (perpendicular to the c � axis), which makes the elec-tronic structure of La3TiX5 three-dimensional like. The calculationsshow a three-dimensional conducting behavior for both La3TiP5and La3TiAs5 although they have a strong 1D structure. In theprevious work about La3TiSb5, the calculation for the band struc-ture of the [TiSb5]9- substructure gave a well-defined 1D con-ducting characteristic if the effect of La atoms on the conductingproperties was ignored [25]. When considering the contribution ofLa atoms, the later calculations displayed that La3TiSb5 is a 3Dmetallic band structure. This is means that the addition of a La atomto the calculations substantially changes the electronic structure,

susceptibility c versus temperature with the applied magnetic field 1 T for La3TiP5 andmperature. The blue line is the corrected magnetic susceptibility. (For interpretation ofthis article).

Fig. 6. The calculations of band structure and partial DOS for (a) La3TiP5 and (b)La3TiAs5. (c) The La 5d state for both samples.

L. Duan et al. / Journal of Alloys and Compounds 831 (2020) 1546976

which suggests that La3þ ions play a key role for La3TiSb5 being a 3Dmetal [26]. In our calculations for La3TiX5 (X ¼ P and As), it wasfound that the bands near the Fermi level are filled with the elec-trons derived from La-5d, Ti-3d and P-3p (As-4p) orbital. In addi-tion, the P-3p (As-4p) orbitals are hybrid with both La-5d and Ti-3dorbitals, which makes the conducting paths in the ab-plane andalong the c axis.

In the plots of PDOS, we can see that although the contributionof La to the DOS near the Fermi level is much less than that of Ti andP/As, it cannot be non-negligible as same as La3TiSb5. It indicatesthat, even in La3TiP5, the La3þ ions are not perfectly ionic and bridgethe conducting chains to cause the 3D metallic behavior. However,compared with La3TiAs5, the spectral weight of La 5d state inLa3TiP5 shifts toward high energy and the DOS near the Fermi levelis suppressed, as shown in Fig. 5(c), which indicates that moreelectrons derived from La-5d are lost. Thereby, the La3þ ions inLa3TiP5 becomemore ionic than that in La3TiAs5. This result leads toa weaker connection between the conducting chains and results inthe increase of resistivity in La3TiP5, as has been observed inFig. 5(a). Finally, we calculated the localized moment of Ti atoms forboth La3TiP5 and La3TiAs5. It shows that the localized moment iszero, which suggests that the small effective moment obtainedfrom susceptibility should come from paramagnetic impurities.

4. Conclusion

The compounds La3TiAs5 and La3TiP5 are the first ternaryphosphide and arsenide in the RE-Ti-X systems, which are syn-thesized under high pressure and high temperature conditions. Thestructure consists of face-sharing TiX6 octahedral chains and X-chains along the c axis and these chains are bridged by La atoms.Both the compounds are Paul paramagnetic metals. The calcula-tions of band structure and DOS indicate that both La3TiAs5 andLa3TiP5 are metallic with a 3D characteristic. In the system ofLa3TiX5, when X varies from Sb to P, the ionic component of La3þ

ions increases, which results in the decrease of the electron hop-ping between the conducting chains and thereby the increase ofresistivity.

Declaration of competing interest

The authors declare that they have no known competingfinancial interests or personal relationships that could haveappeared to influence the work reported in this paper.

CRediT authorship contribution statement

Lei Duan: Formal analysis, Investigation, Data curation, Writing- original draft, Writing - review & editing, Visualization. JunZhang: Formal analysis, Data curation, Writing - original draft,Visualization. Xiancheng Wang: Conceptualization, Formal anal-ysis, Writing - original draft, Writing - review & editing. JianfaZhao: Data curation, Visualization. Lipeng Cao: Data curation,Visualization. Wenmin Li: Data curation, Visualization. ZhengDeng: Formal analysis, Writing - original draft. Runze Yu: Formalanalysis, Supervision. Zhi Li: Methodology, Software. ChangqingJin: Conceptualization, Project administration, Formal analysis,Writing - original draft, Resources.

Acknowledgments

We greatly appreciate the support of the National Key R&DProgram of China and the Natural Science Foundation of Chinaunder grant no. 2018YFA0305700, 2017YFA0302900, 11974410 and11534016.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.jallcom.2020.154697.

L. Duan et al. / Journal of Alloys and Compounds 831 (2020) 154697 7

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