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Synthesis, Structure, and Chemical Pressure Effects pubs.acs.org/IC Article ThMnPnN (Pn = P, As): Synthesis, Structure, and Chemical Pressure Effects Fuxiang Zhang, Baizhuo Li, Qingyong Ren, Huican Mao, Yuanhua Xia, Bingfeng Hu, Zichen Liu, Zhicheng Wang, Yeting Shao, Zhifa Feng, Shugang Tan, Yuping Sun, Zhi Ren, Qiang Jing, Bo Liu, Huiqian Luo, Jie Ma, Yuxue Mei,* Cao Wang,* and Guang-Han Cao Cite This: Inorg. Chem. 2020, 59, 2937−2944 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: Mn-based ZrCuSiAs-type pnictides ThMnPnN (Pn = P, As) containing PbO-type Th2N2 layers were synthesized. The crystal and magnetic structures are determined using X-ray and neutron powder diffraction. While neutron diffraction indicates a C-type antiferromagnetic state at 300 K, the temperature dependence of the magnetic susceptibility shows cusps at 36 and 52 K respectively for ThMnPN and ThMnAsN. The susceptibility cusps are ascribed to a spontaneous antiferromagnetic- to-antiferromagnetic transition for Mn2+ moments, which is observed for the first time in Mn-based ZrCuSiAs-type compounds. In addition, measurements of the resistivity and specific heat suggest an abnormal increase in the density of states at the Fermi energy. The result is discussed in terms of the internal chemical pressure effect. ■ INTRODUCTION plane is observed at lower temperatures, accompanied by 3+ 20−22 The prototype compound of iron-based superconductors antiferromagnetic ordering of the local moment on Ln . LaFeAsO structurally consists of alternating stacks of PbO- Most of the Mn-based 1111 compounds exhibit an insulating 2+ 2− 1 behavior with a typical room-temperature resistivity of about type [La O ] and anti-PbO-type [Fe As ] layers. This − 2 2 2 2 104 Ω·cm.23 26 Upon Sr/La substitution in LaMnAsO, the ZrCuSiAs-type (1111) structure is so adaptable that for every 23 crystallographic site there are a number of candidate elements system changes to an antiferromagnetic metal. It is reported 2−6 2+ that the AFM in LaMnPO can also be suppressed under high that can make it stable. For instance, the [La2O2] layer 2+ 4−6 pressures, yet no superconductivity was observed above 1.5 can be replaced by [Ln2O2] (Ln stands for lanthanides), 27,28 Downloaded via INST OF PHYSICS on May 13, 2020 at 04:28:58 (UTC). 2+ 7,8 2+ K. [An2O2] (An = Np, Pu), [Ae2F2] (Ae = alkali-earth metal 9,10 2+ 11 2− Recently, we discovered a new 1111-type Fe-based or Eu), and [Ca2H2] . As for the [Fe2As2] layer, there are also tens of the alternatives, denoted as [T Pn ]2−, where T compound ThFeAsN that superconducts at 30 K without 2 2 external chemical doping.29,30 Although theoretical calculations and Pn represent transition metals and pnictogen elements, 31 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. 2− suggest a striped antiferromagnetic ground state, no magnetic respectively. In the numerous 1111 compounds, the [T2Pn2] 32−35 layers play the key role in determining the diverse physical ordering of the Fe moments was found down to 2.0 K. properties including metallic antiferromagnetism (AFM) in We also synthesized a Ni-based isomorphic compound LnCrAsO,12 insulating AFM in LaMnPnO,13 high-temperature ThNiAsN, which shows the highest superconducting transition 1 36 superconductivity in LaFeAsO − F , itinerant ferromagnetism temperature Tc among the Ni-based 1111-type family. The − 1 x x (FM) in LnCoPnO,14 17 and Pauli paramagnetism (PM) with abnormal behavior of ThFeAsN and ThNiAsN is attributed to 18,19 2+ 29,36 low-temperature superconductivity in LaNiPnO. the internal chemical pressure from the [Th2N2] layers. Mn-based 1111 compounds are of special interest for their Thus, it is of interest to investigate the Mn-based 1111-type 2+ insulating AFM, akin to that of the parent compounds of pnictides with [Th2N2] layers. cuprate high-temperature superconductors. The Neeĺ temper- In this paper, we report the synthesis, structure, and physical properties of ThMnPnN (Pn = P, As). The neutron powder atures (TN) of LnMnPnO are between 230 and 360 K, and the ordered magnetic moment for Mn2+ spins is in the range of − μ 13 3.2 4.2 B. The Mn moments can be understood in terms of Received: November 9, 2019 the half-filling of 3d orbitals, which maximizes Hund’s Published: February 17, 2020 2+ coupling. Below the TN values, the moments on Mn align along the c axis. In the cases of NdMnAsO, PrMnSbO, and CeMnAsO, reorientation of the Mn2+ moment onto the ab © 2020 American Chemical Society https://dx.doi.org/10.1021/acs.inorgchem.9b03294 2937 Inorg. Chem. 2020, 59, 2937−2944 Inorganic Chemistry pubs.acs.org/IC Article diffraction indicates that the antiferromagnetic states are established at room temperature; nevertheless, the magnetic susceptibility still exhibits a Curie−Weiss-like behavior below 300 K. There are cusps at 36 K (ThMnPN) and 52 K (ThMnAsN) on the χ−T curves, suggestive of the second spontaneous antiferromagnetic phase transition for the Mn2+ moments. In addition, the transport measurement indicates a significant enhancement of the conductivity, consistent with the nonzero Sommerfeld coefficients obtained from the heat- capacity measurement. The unique properties of ThMnPnN are discussed in terms of the internal chemical pressure effect. ■ EXPERIMENTAL SECTION Caution! All of the materials and compounds containing Th are radioactive, including ThO2, Th, Th3N4, and ThMnPnN. Precautions with suitable care and protection for handling such substances were followed. Polycrystalline samples of ThMnPnN were synthesized by a conventional solid-state reaction using the powdered chemicals of Figure 1. Rietveld refinement profiles of the powder XRD for Th3N4 and MnAs (or MnP). The Th metal ingot and Th3N4 powders were obtained as described elsewhere.29 The binary products MnAs ThMnPN (a) and ThMnAsN (b). and MnP were prepared using the powder of As (99.999%), red P (99.999%), and Mn (99.99%) at 973 K in evacuated fused-silica tubes. Table 1. Crystallographic Parameters for ThMnPnN Determined by Rietveld Refinement of Powder XRD at 300 A stoichiometric mixture of Th3N4 and MnAs/MnP (Th:Mn:Pn = a 1:1:1) was cold-pressed into pellets and loaded into boron nitride K crucibles (the crucible was heated to 773 K overnight in air to remove absorbed water in advance). Then the crucibles were sealed in ThMnPN ThMnAsN evacuated fused-silica tubes and heated at 1373 K for 60 h. The solid- space group P4/nmm P4/nmm state reaction was repeated with intermediate regrinding. a (Å) 4.0301(3) 4.0818(7) ff Powder X-ray di raction (XRD) was carried out at room c (Å) 8.6841(7) 8.947(2) temperature on a PANalytical X-ray diffractometer (model EMPYR- α Rwp (%) 6.08 8.30 EAN) with monochromatic Cu K 1 radiation. The crystal structure R (%) 4.82 6.33 data were obtained by Rietveld refinement using the step-scan XRD p χ2 1.06 2.11 data with 20° ≤ 2θ ≤ 120° for all of the samples. Neutron powder diffraction experiments were carried out on the High-Resolution HPn (Å) 1.3978 1.5472 − − Neutron Diffractometer at the Key Laboratory of Neutron Physics, Pn Mn Pn angle (deg) 110.49 105.68 ff Institute of Nuclear Physics and Chemistry, China Academy of atom Wycko xy z Uiso Engineering Physics. About 5 g of high-purity sample was used in the ThMnPN ff neutron di raction for both compounds (sealed in a V can). The Th 2c 0.25 0.25 0.13586(5) 0.0021(1) λ wavelength of the neutron was = 1.8846 Å. The scattering data were Mn 2b 0.75 0.25 0.5 0.0041(5) collected at 4 and 300 K by covering the scattering angle in the range P 2c 0.25 0.25 0.6610(4) 0.0051(7) 2θ =8−145°. The XRD and neutron diffraction patterns were refined with the Rietveld method using the programs RIETAN-FP and N 2a 0.75 0.25 0 0.008(3) FullProf,37 respectively. The structural parameters were obtained by ThMnAsN assuming that the occupancy was the same as the chemical Th 2c 0.25 0.25 0.13091(8) 0.0015(2) composition.33 Magnetic measurements were performed on a Mn 2b 0.75 0.25 0.5 0.0073(9) Quantum Design Magnetic Property Measurement System (MPMS- As 2c 0.25 0.25 0.6729 (2) 0.0037(7) XL5). The temperature-dependent resistivity was measured using a N 2a 0.75 0.25 0 0.014(5) standard four-terminal method on a Cryogenic Mini-CFM measure- aH represents the pnictogen height relative to the Mn plane. ment system equipped with a Keithley 2400 digital sourcemeter and a Pn Keithley 2182 nanovoltmeter. Measurement of the specific heat was performed on a Quantum Design Physical Property Measurement System (PPMS-9) using a thermal relaxation method. structure of the MnPn block layers, characterized by the change of the Pn height as well as the 2-fold Pn−Mn−Pn bond angles. Table 2 compares the cell parameters of the related ■ RESULTS AND DISCUSSION 1111-type pnictides containing Th2N2 or La2O2 layers. For the Figure 1 shows the powder XRD patterns and their Rietveld Fe-based pnictides LaFeAsO and ThFeAsN, while their a axes refinement profiles for ThMnPnN (Pn = P, As) polycrystalline are almost identical, the c axis of ThFeAsN is distinctly shorter samples. No extra XRD peaks can be observed in the pattern, than that of LaFeAsO, suggesting the existence of an additional indicating that our samples are phase-pure. Table 1 lists the uniaxial chemical pressure along the c axis in ThFeAsN.29 refined crystal-structure parameters.
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