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Synthesis and Electron Doping of TiNCl
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Layered metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) have two polymorphs, including α- and β-forms, which have the FeOCl and SmSI structures, respectively. These compounds are band insulators and become metals and show superconductivity after electron doping by intercalating alkali metals between the layers. The superconductivity of β-form had been extensively characterized from decades ago, but it is not easy to consistently interpret all experimental results using conventional phonon-mediated Bardeen–Cooper–Schriefer mechanisms. The titanium compound TiNCl crystallizes only in the α-form structure. TiNCl also exhibits superconductivity as high as ~16 K after electron doping by intercalating metals and/or organic basis. It is important to compare the superconductivity of different M–N networks. However, α-form compounds are vulnerable to moisture, unlike β-form ones. The intercalation compounds are even more sensitive to humid air.

α-form layered nitride halides electron doping Synthesis
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    1. Introduction

    Transition metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) belong to a characteristic class of layered materials that exhibits superconductivity by carrier doping [1][2][3]. They have two polymorphs, including α- and β-forms, which have the FeOCl and SmSI structures, respectively [4][5]. These compounds are band insulators and can be intercalated with alkali or alkaline earth metals to significantly modify the physical properties without changing their basic crystal structures. Alkali atoms with/without organic molecules are intercalated into the van der Waals gap between two adjacent X layers with the realignment of MNX layers.
    The β-form polymorph contains double honeycomb-like MN layers sandwiched by halogen layers. Band-insulating β-HfNCl is electron-doped and becomes metallic through intercalation [2][4]. The compound shows superconductivity at a transition temperature (Tc) above 25.5 K in Li and tetrahydrofuran (THF) cointercalated compound of β-HfNCl [2][4][6]; this family of materials is categorized as high-Tc superconductors. The superconductivity can also be induced by carrier doping with an electronic-double-layer transistor [7][8][9][10][11]. The Tc enhancement has been reported with reducing carrier density [12][13] and the partial substitution M site [14]. Electronic structural studies have shown that alkali-metal intercalation adds quasi-two-dimensional electronic states near the Fermi level (EF), which are hybridized states of Hf 5d and N 2p orbitals [15][16][17][18][19]. The absence of isotope effect, extremely large superconducting gap value 2Δ/kBTc, enhancement of Tc for lower carrier concentration, anomalous doping dependence in anisotropy of superconducting gap, and strong suppression of the coherence peak in nuclear spin-lattice relaxation rate strongly suggest an exotic superconducting state in this material series [13][20][21][22][23][24][25][26][27][28]. With the extensively characterized superconductivity in β-form MNX, it is not easy to consistently interpret all experimental results using conventional phonon-mediated Bardeen–Cooper–Schrieffer (BCS) mechanisms [29].

    2. Conventional Preparation

    For the synthesis of MNX, the tetrahalides of titanium, zirconium, and hafnium are reacted with ammonia following the generic equation:
    MX4 + 4NH3MNX + 3NH4X
    In the case of M = Zr, Hf in the β-form, Yamanaka has efficiently developed the method using a reaction of metal or metal hydride powders with NH4X [3]. The one-step method in a vacuum-sealed glass tube can also be adopted in a small-scale synthesis of less than ~200 mg [2].
    However, in the case of M = Ti, especially TiNCl, the easy process cannot be used. In the reaction between Ti and NH4Cl, TiH2 is first formed at about 400 °C or lower. Above the temperature at which TiH2 can be decomposed, TiNCl is not formed, but TiN is formed instead. Then, the ammonia (NH3) gas should be passed over titanium tetrachloride (TiCl4) in a vertical Pyrex glass reaction cell. The reactant becomes black solid after being self-heated by the heat evolved from the intense exothermic reaction. Due to the high reactivity of TiCl4 with NH3, the reaction may lead to molecules and complexes of several manners [30]. TiCl4 ammonolysis is energetically favorable [30][31]. TiCl4 forms ammonia-incorporated complexes with the TiCl4·xNH3 gross composition, and heating the complexes in ammonia-rich atmosphere results in stepwise amination of TiCl4 to form TiCl(NH2)3 in the final step [32][33][34]. The ammonia complexes and aminated TiCl4 were especially the main products at low temperatures below 400 °C [32], while TiN is obtained at higher temperatures. The resulting solids were continuously heated in the same reaction cell at 400 °C for 3–5 h under the ammonia gas stream. Ammonium chloride (NH4Cl) crystals were grown on the upper part of the reaction cell.
    The TiNCl precursor and other related compounds (crude mixture) were formed at the bottom of the glass cell. The by-products necessitate a next purification step. The crude mixture was vacuum-sealed in a Pyrex glass tube with a small amount of NH4Cl and put into a horizontal furnace with a temperature gradient for one to two weeks. Then, it was purified into highly crystalline TiNCl by chemical transport using NH4Cl. The end of the glass tube containing the mixture was maintained at 380 °C in the lower temperature zone, and the other end of the glass tube was placed in the higher temperature zone. Purified TiNCl was obtained at the higher temperature zone [35]. TiNCl starts to decompose to TiN at temperatures above 400 °C [36][37]. Once it decomposes into TiN with trivalent titanium, it is hard to return to tetravalent TiNCl. Then, it should be maintained at a temperature as low as possible above 400 °C to obtain impurity-free samples.
    Assuming the same reaction as the case of ZrNCl [38], the essential reaction in the chemical vapor transport of TiNCl is expressed as follows:
    TiCl4(g) + NH3(g) → TiCl3(NH2) + HCl → TiNCl(s) + 3HCl(g) (>~400 °C)
    Since HCl is not easy to handle in this preparation, it is used as a transport agent by utilizing the dissociation of NH4Cl into NH3 and HCl at high temperatures. The actual reaction is then expressed as
    TiCl3(NH2) + NH4Cl → TiNCl(s) + NH3(g) + HCl(g)
    After these reactions, it can be confirmed the purity of the obtained TiNCl by the Rietveld refinement and magnetization measurement with the negligible diamagnetic contribution of TiN below 5 K.

    3. Preparation Using Sodium Amide

    In a study on a superconducting mechanism, the isotope effect on Tc provides crucial evidence of whether electron–phonon interaction is responsible for superconductivity. Considering the above conventional synthesizing process of TiNCl, retaining the 15N isotope using NH3 gas as a nitrogen source is extremely difficult or almost impossible realistically. However, if the TiNCl precursor could be formed using a solid reagent other than gas NH3 as a nitrogen source in a closed reaction cell, an essential technique for synthesizing 15N substituents would be developed. It could be led not only to elucidate the superconducting mechanism of TiNCl but also to investigate the isotope effect of other metal nitride superconductors.
    NaNH2 is a highly reactive solid nitrogen source used for combustion reaction with 3d transition metal chlorides to obtain metal nitrides [39][40]. The reactions are thermodynamically driven by a metathesis reaction involving the formation of stable NaCl. The NaCl does not contribute to chemical transport; if the reaction with TiCl4 can be controlled, TiNCl may be obtained in the sealed glass tube.
    When the sealed glass tube containing TiCl4 and NaNH2 was gradually heated up to 150 °C in 12 h and then kept at this temperature for 1-2 days, the infrared spectroscopy, thermogravimetry, and X-ray diffraction measurements of the resulting solids suggested that it contains TiCl3(NH2), TiCl2(NH2)2, and TiCl(NH2), together with TiCl4·xNH3 [41]. Namely, the reaction at this process can be written as the equation TiCl4 + NaNH2 → TiCl3(NH2) + NaCl in the simplest case. After an important process of removing HCl from the reaction system, researchers can obtain TiNCl with the whole equation in the simplest case written in TiCl4 + NaNH2 → TiCl3(NH2) + NaCl → TiNCl + NaCl + 2HCl.
    The procedures to obtain TiNCl using NaNH2 can be summarized as the following. (1) The sealed mixture of TiCl4 + NaNH2 is sufficiently reacted around 160 °C to obtain preliminary reacted solids. (2) Open the seal and purge unreacted TiCl4 from the glass tube. Then, evacuate the preliminary reacted solids again with the elevated temperature at 180 °C to remove HCl from TiCl3(NH2) and/or other aminated compounds. (3) The heating and evacuation should be stopped before decomposition to TiN occurs and sealed again into a glass tube. (4) The glass tube sends to chemical transport to obtain highly crystalline TiNCl [41].

    4. Electron Doping by Intercalation

    Metal intercalation into TiNCl was developed from an electrochemical process as a cathode material [42]. First, superconducting TiNCl has been achieved by an organic Lewis base pyridine (Py) and alkali metal intercalation [3]. The alkali-metal-intercalated TiNCl was prepared using reactions with alkali metal azides at elevated temperatures of 280–350 °C in a vacuum. The intercalation was carried out in the following manner:
    TiNCl + (x + y) AN3 > AyTiNCl1-x + xACl + 3(x + y)/2N2
    This reaction was not mild, and partial deintercalation of chlorine atoms occurred simultaneously. The superconducting volume fractions (VF) were as low as 0.1–30%. The samples may have suffered from serious structural damage.
    A chemical intercalation method using alkali-metal naphthalene solutions in THF (A-Naph/THF; A = Li, Na, K, Rb) has been used for β-phase for a long time [43]. This method has dramatically improved the low VF problem [44]. Organic molecules such as THF can be cointercalated into TiNCl at room temperature. The THF-cointercalated phase was changed into the uncointercalated compound AxTiNCl by evacuation. The sample thus obtained showed a shielding VF close to 100% and a clear high onset superconducting transition. By this method, the same α-phase TiNBr has also succeeded in being a superconductor [45]. On the other hand, α-HfNBr isomorphous with TiNCl did not show superconductivity in the same method. The resistivity was larger than ~107 Ὠ cm; the doped electrons seem to be localized on the reduced Hf sites [46]. It should be noted that only Py intercalated TiNCl showed superconductivity. It appears the combination of guest donors and the amount of doping are important to realize the superconductivity of the α-form MNX.
    The Tc changes with the basal spacing d along the c axis of the intercalation compound.  The Tc decreases with the increasing d [44]. This fact is in sharp contrast to the observation in β-form, where Tc increases linearly with d in electron-doped ZrNCl and HfNCl [25][47]. The data fit on a linear line with passing through the origin. This fact suggests that the Coulomb interlayer coupling in the pairing mechanism is important in this system [44]. Another study on the α-TiNBr has demonstrated a similar linear relation on the Tc vs. 1/d [45]. Harshman et al. reported that the linear dependence is readily explained, giving the interlayer Coulomb interaction model and structure [48]. It is also a future task to experimentally observe the effect of the interlayer interaction on the electronic structure.

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      Tanaka, M. Synthesis and Electron Doping of TiNCl. Encyclopedia. Available online: https://encyclopedia.pub/entry/22299 (accessed on 06 February 2023).
      Tanaka M. Synthesis and Electron Doping of TiNCl. Encyclopedia. Available at: https://encyclopedia.pub/entry/22299. Accessed February 06, 2023.
      Tanaka, Masashi. "Synthesis and Electron Doping of TiNCl," Encyclopedia, https://encyclopedia.pub/entry/22299 (accessed February 06, 2023).
      Tanaka, M. (2022, April 26). Synthesis and Electron Doping of TiNCl. In Encyclopedia. https://encyclopedia.pub/entry/22299
      Tanaka, Masashi. ''Synthesis and Electron Doping of TiNCl.'' Encyclopedia. Web. 26 April, 2022.
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