Correlation between Chemical and Physical Pressures on Charge

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Correlation between Chemical and Physical Pressures on Charge Bistability in [Pd(en)2Br](Suc‑Cn)2·H2O

Shohei Kumagai,†,‡ Shinya Takaishi,*,† Hiroaki Iguchi,† Brian K. Breedlove,† Takuya Kaneko,§ Hiroshi Ito,*,§ Shin-ichi Kuroda,§,¶ and Masahiro Yamashita*,†,⊥,∥ †

Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan § Department of Applied Physics, Graduate School of Engineering, Nagoya University, Chikusa Ward, Furocho, Nagoya 464-0814, Japan ⊥ Advanced Institute of Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ∥ School of Materials Science and Engineering, Nankai University, Tianjin 300350, China S Supporting Information *

Recently, we synthesized the first Pd3+−Pd3+ Mott−Hubbard complexes, [Pd(en)2Br](Suc-Cn)2·H2O [en = ethylenediamine, Suc-Cn− = dialkyl sulfosuccinate, and n = number of carbon atoms within the alkyl chain; hereafter abbreviated as PdBrCn (n = 5, 6, 7, 8, 9, and 12)].10 Figure 1a shows a crystal structure of PdBrC5. PdBrC5 exhibits charge bistability accompanied by a first-order phase transition between a Pd2+−Pd4+ CDW state [high-temperature (HT) phase; upper panel of Figure 1b] and a Pd3+−Pd3+ MH state [low-temperature (LT) phase; lower panel of Figure 1b] at a CDW-to-MH phase-transition

ABSTRACT: Hydrostatic (physical) pressure effects on the electrical resistivity of a bromido-bridged palladium compound, [Pd(en)2Br](Suc-C5)2·H2O, were studied. The charge-density-wave to Mott−Hubbard phase transition temperature (TPT) steadily increased with pressure. By a comparison of the effects of the chemical and physical pressures on TPT, it was estimated that the chemical pressure by unit alkyl chain length, i.e., the number of carbon atoms in the alkyl chains within the counterion, corresponded to ca. 1.3 kbar of the physical pressure.

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istability is a key property in functional materials. In the solid state, for example, charge-transfer phenomena accompanied by a phase transition have been frequently studied and used for switching materials.1 However, it is still difficult to introduce charge bistability in material design. Quasi-one-dimensional halogen-bridged metal complexes (MXchain complexes) are attractive target materials for realizing charge bistability in the solid state. In MX-chain complexes, the dz2 orbital of metal ions (M = Ni, Pd, and Pt) and the pz orbital of bridging halide ions (X = Cl, Br, and I) form onedimensional electron bands. This system is theoretically recognized as a Peierls−Hubbard system, in which the transfer integral (t), on-site Coulomb repulsion (U), Coulomb repulsion between nearest-neighbor sites (V), and electron− lattice interaction (S) compete or cooperate with each other.2 It is known that M3+−M3+ averaged valence states [or Mott− Hubbard (MH) states] are stabilized when U > S.2a From MH states, various physical properties, such as gigantic third-order nonlinear-optical susceptibility,3 negative differential electrical conductivity,4 electrostatic carrier doping effects,5 and ultrafast photoinduced phase transitions, have been reported so far.6 Nickel compounds have been the focus of these studies because they are in MH states7 or class III compounds in the Robin− Day classification.8 On the other hand, palladium and platinum compounds have been known to show M2+−M4+ mixed-valence states [or charge-density-wave (CDW) states],9 in which S > U. Complexes in this state are categorized as class II compounds in the Robin−Day classification.8 © XXXX American Chemical Society

Figure 1. (a) Crystal structure of PdBrC510 (some hydrogen atoms are omitted for clarity). Color code: black, C; white, H; brown, Br; blue, N; red, O; gray, Pd; yellow, S. (b) Electronic state (red arrows indicate electron spins on the dz2 orbital). (c−f) Physical properties of PdBrC5 displaying the temperature (T) dependences of the Pd−Pd distance, spin susceptibility (χ), charge-transfer energy (ECT), and electrical resistivity (ρ), respectively. The circle and triangular marks indicate cooling and heating processes, respectively. Parts c−f are previously reported data,10 which are herein rearranged. Received: October 6, 2017

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DOI: 10.1021/acs.inorgchem.7b02566 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

report (Figure 1f).10 Upon cooling, ρ steeply dropped just above TPT and steadily increased below TPT. Thus, TPT is estimated to be 205, 230, 256, and 276 K at Pinit = 1 × 10−3, 1, 2, and 3 kbar, respectively, from an inflection point of ρ (Figure 2b). Here, the effective P values at TPT were converted from 1, 2, and 3 kbar to 0.56, 1.77, and 2.86 kbar, respectively, based on the previous report (the details are described in the Supporting Information).15 Hence, the increase in TPT per unit P was approximated as 22.2 ± 3.5 K kbar−1. Figure 2c shows an Arrhenius plot of ρ in the LT phase. As can be seen, the Arrhenius law could be used to fit the observed ρ values. In addition, the activation energy (Ea) slightly decreased with an increase in P from Ea = 1.5 × 102 meV at 1 × 10−3 kbar to Ea = 1.2 × 102 meV at Pinit = 3 kbar. This phenomenon can be explained by considering the origin of the band gap. Parts d and e of Figure 2 show the band structures of the MH and CDW states of PdBrC5, respectively. The valence and conduction bands of the MH state are composed of the lower- and upperHubbard (LH and UH, respectively) bands formed by the 4dz2 orbital of the Pd3+ ions. Here, the band gap is determined by U, where the contribution of V is ignored because V is much smaller than U.2a,10 Basically, U is determined by the elemental character of the ion and independent of the surrounding environments. Thus, Ea only slightly changes because of the physical pressure. The slight decrease in Ea in PdBrC5 is probably due to the increment in t upon the shortening of the nearest-neighbor Pd···Pd distance (dPd−Pd) under P, which leads to better electronic communication between the neighboring palladium sites. This finding indicates that U is dominant in the MH state of this compound. On the other hand, above TPT (in the HT phase), ρ highly depended on P and did not obey the Arrhenius law. This phenomenon can be qualitatively explained by the band structure of the CDW state. In the CDW state, the valence and conduction bands are composed of the 4dz2 bands of Pd2+ and Pd4+ species, respectively. The band-gap energy is equal to 2S − U, where t and V are ignored.2a,10 In the previous paper, it was concluded that S remarkably decreased with a decrease in dPd−Pd, whereas U scarcely changed.10 Thus, Ea exhibited a large T dependence, resulting in a deviation from the Arrhenius law. Also, the decrease in ρ under P observed in the current work can be understood by the decrease in dPd−Pd. Next, we discuss the T and P dependences of dPd−Pd in detail. In our previous report, we show that the MH and CDW states are formed when dPd−Pd is shorter and longer than the threshold Pd···Pd distance (dth), respectively.10 This indicates that TPT is the T at which dPd−Pd crosses dth. It should be noted that dth has been typically considered to be 5.26 Å in the previous papers,10,11,16 which is the middle value in the discontinuous change of dPd−Pd in the range of 5.25−5.27 Å. Because dPd−Pd can continuously decrease down to 5.27 Å in the CDW state, we define dth as 5.27 Å in this work. Then, the degree of contraction of dPd−Pd caused by P (ΔdPd−Pd) can be estimated by subtracting dth (=5.27 Å) from dPd−Pd under ambient P at each TPT (Figure 3a). In Figure 3b, the estimated ΔdPd−Pd is plotted as a function of P. Averaged ΔdPd−Pd per unit P was approximated to be 0.012 Å unit kbar−1 (=0.23% kbar−1). Tanino et al. have reported a hydrostatic pressure effect on the lattice shrinkage in the analogous MX-chain complex [Pt(etn)4][PtCl2(etn)4]Cl4·4H2O (etn = ethylamine), which is known as Wolfram’s red salt.17 Lattice shrinkage of this compound has been estimated to be 0.37% kbar−1, which is larger than that for PdBrC5 in this work. This is probably

temperature (TPT) of 206 K. Parts c−f of Figure 1 show the temperature (T) dependence of the physical properties of PdBrC5, all of which display discontinuous points near TPT. The origin of this phase transition is explained by a shortening of the intrachain Pd···Pd distance induced by the attractive forces between the alkyl chains of Suc-Cn−, which acts as the chemical pressure. The interest in the MH state of palladium compounds is different from that of nickel ones. Because the valence orbital of Pd 4d is larger than that of Ni 3d, U in the palladium compounds is smaller than that in the nickel ones. As a result, the energies of U and S are comparable, leading to fluctuations in the electronic states. On the basis of the chemical pressure concept and PdBrCn, an analogous palladium compound, which undergoes a phase transition at a higher TPT than the PdBrCn system does, has been reported.11 By shortening the Pd···Pd distance, a stable MH state in a palladium complex has finally been realized, and the complex does not undergo a phase transition to the CDW state.12 In this paper, we studied the dependence of TPT of PdBrC5 on the physical pressure by measuring the electrical resistivity under hydrostatic pressure and the relationship between the chemical and physical pressures in the series of PdBrCn. To date, such systematic investigations on the correlation between the chemical and physical pressures by chemical modification of molecular species have been reported for only Cu(R2DCNQI)2 derivatives (R2-DCNQI = 2,5-substituted dicyanobenzoquinonediimine).13 As for the use of alkyl chains to apply chemical pressure, for instance, tetrathiafulvalene derivatives have been reported.14 However, the correlation has not been clarified. Therefore, this work gives an important insight into material design utilizing alkyl chains as a source of chemical pressure. Figure 2 shows the T dependence of ρ for PdBrC5 under hydrostatic pressure (P) with initially applied pressures (Pinit) of 1 bar (ambient pressure; 1 × 10−3 kbar) and 1, 2, and 3 kbar. ρ at ambient pressure agrees with that shown in the previous

Figure 2. (a) Temperature dependence of the electrical resistivity of PdBrC5 at Pinit = 1 × 10−3, 1, 2, and 3 kbar. (b) TPT of PdBrC5 as a function of the hydrostatic pressure. (c) Arrhenius plot of the electrical resistivity in the LT phase. Gray solid lines were fitted. (d and e) Schematic illustrations of the band structures of the CDW and MH states, respectively, in PdBrC5. B

DOI: 10.1021/acs.inorgchem.7b02566 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

transition. In PdBrC5, TPT and the applied hydrostatic pressure have a linear correlation in relation to TPT and n in PdBrCn. From a comparison of the n dependence of TPT in a series of PdBrCn with the current results, we concluded that the chemical pressure effect per n corresponded to a physical pressure of ca. 1.3 kbar.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02566. Experimental details, determination scheme of effective physical pressure at low temperatures, and photograph of the PdBrC5 sample (PDF)

Figure 3. (a) Temperature dependence of the Pd···Pd distance (dPd−Pd) in PdBrC5 (hollow circle), where the data were extracted from the single-crystal X-ray diffraction analysis previously performed.10 The black solid line was fitted to the temperature-dependent dPd−Pd value. The black broken line indicates the position of dPd−Pd = 5.27 Å ≡ dth. Magenta dots and broken lines link TPT under pressure with the line of dth. Blue arrows indicate the difference between dPd−Pd under 1 × 10−3 kbar and that under respective pressures, thus defining ΔdPd−Pd. dPd−Pd at 1 × 10−3 kbar was estimated from the fitted line (b) ΔdPd−Pd in PdBrC5 (estimated from part a) as a function of the hydrostatic pressure.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.T.). *E-mail: [email protected] (H.I.). *E-mail: [email protected] (M.Y.). ORCID

because PdBrC5 originally receives the chemical pressure, and thus a stronger P is necessary for further lattice shrinkage. Finally, the chemical and physical pressure effects in PdBrCn are compared. Figure 4 shows TPT of PdBrC5 as a function of P

Shohei Kumagai: 0000-0002-1554-054X Shinya Takaishi: 0000-0002-6739-8119 Hiroaki Iguchi: 0000-0001-5368-3157 Present Addresses ‡

S.K.: Department of Advanced Materials Sciences, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan. ¶ S.-i.K.: Toyota Physical and Chemical Research Institute, 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI [Grant(A) 26248015 and Grant(C) 16K05713], CREST (JST), and the Tohoku University Molecule and Material Synthesis Platform in Nanotechnology Platform project sponsored by the Ministry of Education, Culture, Sports, Science and Technology, Japan.

Figure 4. Plot of TPT as a function of the hydrostatic pressure in PdBrC5 (red triangle; upper scale) and n of PdBrCn at ambient pressure (green circle; lower scale). The black broken line is a guide for the eyes.

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(red triangles; upper scale) and that of PdBrCn (n = 5, 6, 7, and 8) as a function of the alkyl chain length, n, under ambient pressure (green circles; lower scale).10 In the latter case, TPT steadily increases with an increase in n due to the stronger chemical pressure. Moreover, both appear to show linear relationships. In the case of the chemical pressure effect, TPT increased by 29.0 ± 3.3 K per value of n. As for the physical pressure, TPT increased by 22.2 ± 3.5 K per unit pressure (kbar). Hence, we concluded that the correlation between the chemical and physical pressures was ca. 1.3 kbar per increment of n for PdBrCn. In conclusion, we studied the hydrostatic pressure effects on the electrical resistivity of a bromide-bridged palladium chain complex, PdBrC5. TPT steadily increased with an increase in the hydrostatic pressure due to shortening of the Pd···Pd distance, which is the origin of the CDW-to-MH phase

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DOI: 10.1021/acs.inorgchem.7b02566 Inorg. Chem. XXXX, XXX, XXX−XXX