Hua
Lin
a,
Jin-Ni
Shen
ab,
Yong-Fang
Shi
a,
Long-Hua
Li
a and
Ling
Chen
*a
aKey Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People's Republic of China. E-mail: chenl@fjirsm.ac.cn; Tel: +(011)86-591-63173131
bUniversity of Chinese Academy of Sciences, Beijing 100039, People's Republic of China
First published on 21st January 2015
New quaternary selenides, Cs[RE9Mn4Se18] (RE = Ho–Lu), have been synthesized by high temperature solid state reactions of elements with a reactive CsCl flux. These compounds adopt the BaV13O18-structure type in the trigonal space group R (no. 148) with a = 17.4867(8)–17.263(2) Å, c = 9.7996(8)–9.681(2) Å and Z = 3. The major structure motif is the 3D network that is constructed by octahedra of MSe6 (centered by disordered Mn and RE) and RESe6, in which closed cavities of cuboctahedra Cs@Se12 are embedded. This work extends the previously established empirical A/M-structure correlation in the ternary A/RE/Q system into quaternary A/RE/TM/Q, and A/M = 0.08 (A = the number of alkali metals, M = the sum of the numbers of RE and TM metals) represents the minimum ending value. The syntheses, properties, and theoretical analyses of the title compounds are also discussed.
In this work, based on the systematic structure study of all available examples, we find that, in addition to ternary systems, the heterometallic anionic moiety in quaternary A/RE/TM/Q (TM = transition metals) systems also shows similar empirical A/M-correlation: as the A/M ratio decreases, the channel size decreases (note that M is now the sum of the numbers of RE and TM metals) (Fig. 1, right). Examples are ARETMQ3,11 ARE2TM3Q5,1 CsSc3Cu2Q6,12 K2CeAg3Te4,13 A3RE4Cu5Q10,1etc. On going from K2CeAg3Te4,13 K3Dy4Cu5Te10,1 to CsSc3Cu2Te6,12 as A/M decreases from 0.5, 0.33 to 0.2, the number of A atoms accommodated per channel drops from 4, 3 to 1 indicating the shrink of the channel. Note that TM and RE atoms usually have different local coordination behaviour, tetrahedron vs. octahedron, thus the corresponding monometallic and heterometallic anionic moieties differ in the structure considerably. Therefore, the A/M value may not be numerically equal for the corresponding ternary and quaternary structure-class (Fig. 1). For example, in the case of the 2D layered structure class, ternary compounds have an A/M of 1.0, but for quaternary it is 0.5. At A/M = 0.33, both ternary and quaternary compounds will be channel structures, but the former accommodates less A atoms per channel than the latter. Nevertheless, the principle A/M ratio–structure relationships coincide, and the channel size in both ternary and quaternary smoothly decreases until the A/M value drops to 0.2 (Fig. 1). Very interestingly, recent work shows that in the ternary system, when A/M = 0.14, the anionic moiety is no longer an open channel structure, instead it is a 3D network embedded with a closed cavity, such as in the CsLu7Q11 network, every two Cs atoms are captured into a dual-tricapped cube, Cs2@Se18.6 But if A/M in the quaternary system is further reduced, how the structure will change is yet unknown. Since low A/M ratio means low concentration of the reducing agent, it is also a great experimental challenge.
In this manuscript, we discover an even smaller A/M ratio of 0.08 in five new quaternary compounds, Cs[RE9Mn4Se18] (RE = Ho–Lu), with Cs atoms being captured into a smaller closed cavity of the Se12 cuboctahedron. The syntheses, crystal structure analyses, properties as well as theoretical analyses are reported.
After numerous explorations on the experimental conditions including the starting reactant, loading ratio, and annealing temperature, the optimal synthesis conditions were established as loading a mixture (about 300 mg in total) of CsCl, RE, Mn and Se in a molar ratio of 2:9:4:18 with a slight excess of CsCl as the reactive flux. The reactants were loaded in a fused-silica tube under vacuum, heated to 1223 K for 2 days, annealed at this temperature for 4 days, and then slowly cooled at 3 K h−1 to 473 K before switching off the furnace. Crystals of good quality were subsequently selected for single crystal X-ray diffraction studies, e.g. Cs[Tm9Mn4Se18] with a cuboctahedral crystal behaviour at room temperature (Fig. 2a). The EDX analyses carried out on several single crystals revealed the presence of Cs, RE, Mn, and Se in a ratio of 1:9:4:18. For example, CsTm9.1(3)Mn4.0(1)Se18.1(2) (Fig. 2b) was close to CsTm9.0(3)Mn4.0(3)Se18 as established by single-crystal diffraction data (Table 1 and Table S1 in ESI†). No other element was detected. The products were washed with distilled water to remove the excess CsCl and chloride by-products, and then dried with ethanol. After such a treatment, title compounds were obtained as pure phases according to the powder XRD patterns shown in Fig. 2c and Fig. S1 in ESI.† These compounds were stable in air for several months.
Formula | Cs[Ho9Mn4Se18] | Cs[Er9Mn4Se18] | Cs[Tm9Mn4Se18] | Cs[Yb9Mn4Se18] | Cs[Lu9Mn4Se18] |
---|---|---|---|---|---|
a R 1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2. | |||||
fw | 3258.31 | 3279.29 | 3294.32 | 3331.30 | 3348.64 |
Crystal system | Trigonal | Trigonal | Trigonal | Trigonal | Trigonal |
Space group | R (no. 148) | R (no. 148) | R (no. 148) | R (no. 148) | R (no. 148) |
a (Å) | 17.4867(8) | 17.441(4) | 17.360(2) | 17.3186(6) | 17.263(2) |
c (Å) | 9.7996(8) | 9.785(3) | 9.724(2) | 9.6972(5) | 9.681(2) |
V (Å3) | 2595.1(3) | 2577.9(2) | 2537.8(5) | 2518.9(2) | 2498.3(4) |
Z | 3 | 3 | 3 | 3 | 3 |
D c (g·cm−3) | 6.255 | 6.337 | 6.467 | 6.588 | 6.677 |
μ (mm−1) | 41.679 | 43.215 | 45.174 | 46.800 | 48.590 |
GOOF on F2 | 1.036 | 1.093 | 1.086 | 1.057 | 1.091 |
R 1, wR2 (I > 2σ(I))a | 0.0285, 0.0653 | 0.0167, 0.0328 | 0.0411, 0.0868 | 0.0247, 0.0527 | 0.0305, 0.0768 |
R 1, wR2 (all data) | 0.0305, 0.0663 | 0.0180, 0.0331 | 0.0459, 0.0888 | 0.0259, 0.0532 | 0.0308, 0.0770 |
Largest diff. peak and hole (e Å−3) | 2.319, −1.379 | 2.250, −1.134 | 4.376, −3.865 | 1.448, −3.361 | 2.259, −2.127 |
Atom | Symmetry | x | y | z | U (eq)b (Å2) | Occu. |
---|---|---|---|---|---|---|
a The RE and Mn were restricted to sum up 100% occupancy at each of M1 and M2 sites. b U (eq) is defined as one-third of the trace of the orthogonalized Uij tensor. | ||||||
CsHo9.0(3)Mn4.0(3)Se18a | ||||||
Cs1 | 3b | 0 | 0 | 0.5 | 0.0437(3) | 1 |
M1 = Ho1/Mn1 | 18f | 0.24085(2) | 0.27636(2) | 0.33554(2) | 0.0086(2) | 0.84(2)/0.16(2) |
M2 = Ho2/Mn2 | 18f | 0.40509(2) | 0.17719(2) | 0.33403(3) | 0.0107(2) | 0.49(2)/0.51(2) |
Ho3 | 3a | 0 | 0 | 0 | 0.0090(2) | 1 |
Se1 | 18f | 0.26808(3) | 0.15976(3) | 0.49798(4) | 0.0102(2) | 1 |
Se2 | 18f | 0.12506(3) | 0.14042(3) | 0.16153(4) | 0.0100 (2) | 1 |
Se3 | 18f | 0.38269(3) | 0.29806(3) | 0.16872(4) | 0.0080(2) | 1 |
CsEr9.0(3)Mn4.0(3)Se18a | ||||||
Cs1 | 3b | 0 | 0 | 0.5 | 0.0424(2) | 1 |
M1 = Er1/Mn1 | 18f | 0.05694(2) | 0.42580(2) | 0.00200(2) | 0.0107(2) | 0.84(2)/0.16(2) |
M2 = Er2/Mn2 | 18f | 0.15606(2) | 0.26151(2) | 0.00124(3) | 0.0110(2) | 0.49(2)/0.51(2) |
Er3 | 3a | 0 | 0 | 0 | 0.0102(2) | 1 |
Se1 | 18f | 0.01538(2) | 0.14024(3) | 0.16183(2) | 0.0104(2) | 1 |
Se2 | 18f | 0.17362(2) | 0.39874(3) | 0.16483(2) | 0.0100(2) | 1 |
Se3 | 18f | 0.28404(2) | 0.24875(3) | 0.16477(2) | 0.0101(2) | 1 |
CsTm9.0(3)Mn4.0(3)Se18a | ||||||
Cs1 | 3b | 0 | 0 | 0.5 | 0.0434(5) | 1 |
M1 = Tm1/Mn1 | 18f | 0.05715(3) | 0.42611(3) | 0.00214(4) | 0.0136(2) | 0.82(2)/0.18(2) |
M2 = Tm2/Mn2 | 18f | 0.15598(4) | 0.26149(4) | 0.00124(6) | 0.0159(2) | 0.51(2)/0.49(2) |
Tm3 | 3a | 0 | 0 | 0 | 0.0155(3) | 1 |
Se1 | 18f | 0.01525(5) | 0.13995(5) | 0.16192(8) | 0.0136(3) | 1 |
Se2 | 18f | 0.17349(5) | 0.39881(5) | 0.16504(8) | 0.0140(3) | 1 |
Se3 | 18f | 0.28447(5) | 0.24910(6) | 0.16443(8) | 0.0126(3) | 1 |
CsYb9.0(3)Mn4.0(3)Se18a | ||||||
Cs1 | 3b | 0 | 0 | 0.5 | 0.0352(2) | 1 |
M1 = Yb1/Mn1 | 18f | 0.05696(2) | 0.42590(2) | 0.00194(2) | 0.0063(2) | 0.81(2)/0.19(2) |
M2 = Yb2/Mn2 | 18f | 0.15606(2) | 0.26151(2) | 0.00124(3) | 0.0078(2) | 0.52(2)/0.48(2) |
Yb3 | 3a | 0 | 0 | 0 | 0.0078(2) | 1 |
Se1 | 18f | 0.01530(3) | 0.13988(3) | 0.16225(4) | 0.0069(2) | 1 |
Se2 | 18f | 0.17347(3) | 0.39897(3) | 0.16543(4) | 0.0070(2) | 1 |
Se3 | 18f | 0.28448(3) | 0.24905(3) | 0.16474(4) | 0.0056(2) | 1 |
CsLu9.0(3)Mn4.0(3)Se18a | ||||||
Cs1 | 3b | 0 | 0 | 0.5 | 0.0377(3) | 1 |
M1 = Lu1/Mn1 | 18f | 0.05690(2) | 0.42596(2) | 0.00181(3) | 0.0107(2) | 0.82(2)/0.18(2) |
M2 = Lu2/Mn2 | 18f | 0.15601(2) | 0.26155(2) | 0.00132(3) | 0.0120(2) | 0.52(2)/0.48(2) |
Lu3 | 3a | 0 | 0 | 0 | 0.0118(2) | 1 |
Se1 | 18f | 0.01530(3) | 0.13967(3) | 0.16247(5) | 0.0109 (2) | 1 |
Se2 | 18f | 0.17355(3) | 0.39925(3) | 0.16547(5) | 0.0108(2) | 1 |
Se3 | 18f | 0.28436(3) | 0.24904(4) | 0.16480(5) | 0.0093(2) | 1 |
RE = Ho | RE = Er | RE = Tm | RE = Yb | RE = Lu | |
---|---|---|---|---|---|
M1–Se2 | 2.7782(5) | 2.7727(6) | 2.7556(9) | 2.7508(4) | 2.7437(6) |
M1–Se1 | 2.8009(5) | 2.7946(7) | 2.7802(9) | 2.7740(4) | 2.7677(6) |
M1–Se2 | 2.8087(5) | 2.8042(6) | 2.7874(9) | 2.7827(4) | 2.7761(6) |
M1–Se3 | 2.8331(5) | 2.8257(7) | 2.8075(9) | 2.7994(5) | 2.7938(6) |
M1–Se2 | 2.8358(5) | 2.8282(7) | 2.8126(9) | 2.8020(5) | 2.7944(6) |
M1–Se3 | 2.8728(5) | 2.8661(7) | 2.8535(9) | 2.8438(4) | 2.8356(6) |
M2–Se1 | 2.7650(6) | 2.7616(7) | 2.752(2) | 2.7480(5) | 2.7446(6) |
M2–Se2 | 2.7720(6) | 2.7693(7) | 2.755(2) | 2.7524(5) | 2.7472(6) |
M2–Se1 | 2.7961(6) | 2.7903(7) | 2.776(2) | 2.7717(5) | 2.7658(6) |
M2–Se3 | 2.8413(5) | 2.8366(7) | 2.824(2) | 2.8186(5) | 2.8110(6) |
M2–Se3 | 2.8516(5) | 2.8459(7) | 2.832(2) | 2.8250(5) | 2.8176(6) |
M2–Se3 | 2.8538(5) | 2.8484(7) | 2.835(2) | 2.8260(5) | 2.8193(6) |
RE3–Se1 × 6 | 2.8192(5) | 2.8120(6) | 2.7944(8) | 2.7879(4) | 2.7785(5) |
Fig. 4 Plots of the unit cell parameters vs. the atomic number for Cs[RE9Mn4Se18] (RE = Ho–Lu) and the red lines represent the least-square fitting. |
Fig. 5 The structure comparison (up), A/M ratio (middle), and Cs-centering polyhedron (bottom) of Cs[RE9Mn4Se18] (left) and Cs[Lu7Se11] (right). |
Selected bond distances of Cs[RE9Mn4Se18] are shown in Table 3. All metal atoms (RE and M) are octahedrally coordinated by six Se atoms with distances of 2.7785(5)–2.8192(5) Å and 2.7437(6) Å–2.8661(5) Å for RE–Se and M–Se, respectively. These bond distances are consistent with those in CsREMnSe3 (RE–Se: 2.8360(5)–2.9325(5) Å and Mn–Se: 2.4996(8)–2.5898(7) Å),19 CsLu7Se11 (Lu–Se: 2.701(2)–2.911(1) Å),6 and Cs2Mn3Se4 (Mn–Se: 2.540(2)–2.576(3) Å).20 The Se–M–Se angles deviate from the ideal values (90° and 180°), ranging from 86.84(7)–94.71(9)°, and 175.9(2)–179.6(2)°, respectively. The Cs centering-cuboctahedron exhibits a 12-fold site symmetry with Cs–Se distances ranging from 3.9904(6) to 4.0846(5) Å, in comparison with those in CsLu7Se11 (3.634–4.133 Å),6 CsTaSe3 (3.996–4.004 Å),21 and CsRe6I3Se8 (3.955–4.076 Å).22 The same Se12 cuboctahedron is also observed in KCd4Ga5S12-type compounds.4b,23
Considering RE and Mn having the same coordination environment, the formula of title compounds can be written as [CsRE9Mn4Se18] ≡ “CsM13Se18”. As shown in Fig. 5, the title compounds and Cs[Lu7Se11]6 are 3D network structures made by metal–Se octahedra, within which cavities of either small cuboctahedra Cs@Se12 or large dual-tricapped cubes Cs2@Se18 are embedded. As the A/M ratio decreases from 0.14 to 0.08, the cavity size decreases from Se18 atoms to Se12. Previous studies suggest that when the A/M ratio drops from 1 to 0.2 (Fig. 1), the anionic moiety varies from infinite 2D layers to open channel structures, of which, the channel size gradually narrows and reaches the minimum at A/M = 0.2.6 The Cs[Lu7Q11] example illustrates that at A/M = 0.14, the open channel has been closed up into individual closed cavities.6 Here, we demonstrate nicely that the cavity size also reduces with the A/M decrease.
The magnetic susceptibility was measured at an applied field of 1000 Oe in the temperature range 2–300 K and shown as χ−1–T plots in Fig. 6. The susceptibility data in the temperature range 40–300 K were fit by a least-squares method to the Curie–Weiss equation χ = C/(T − θ), where χ is the magnetic susceptibility, C is the Curie constant and θ is the Weiss constant. The theoretical total effective magnetic moment can be calculated by the equation μeff(cal.) = [9μeff(RE)2 + 4μeff(Mn)2]1/2μB, and the experimental effective magnetic moment was calculated from the equation μeff(obsd.) = (7.997C)1/2μB.24 The resulting values for C, θ, and μeff are summarized in Table 4. The negative θ value suggests significant antiferromagnetic interactions existing between the magnetic ions. Furthermore, the experimental value of μeff for Cs[Ho9Mn4Se18] is smaller than the theoretical value calculated for Ho3+ and Mn2+. This presumably is attributed to that the 3d electrons of Mn lie in the outermost shell and are responsible for the magnetic behavior of Cs[Ho9Mn4Se18], whereas the electrons of the 4f shell of Ho lie deep inside the ion, within the 5s and 5p shells. Such a result is similar to that in CsTmMnSe3.19 The magnetic interactions should occur mainly between the centers M–M or RE–M (the nearest distance is around 3.90 Å).
Fig. 6 Temperature dependence of the inverse molar magnetic susceptibility (χ−1) of Cs[RE9Mn4Se18] (RE = Ho–Lu). |
C (emu K mol−1) | θ (K) | μ eff (μB) obsd | μ eff (μB) cal. | |
---|---|---|---|---|
Cs[Ho9Mn4Se18] | 133.52 | −10.05 | 32.91 | 33.93 |
Cs[Er9Mn4Se18] | 118.34 | −9.20 | 30.77 | 31.11 |
Cs[Tm9Mn4Se18] | 79.43 | −18.29 | 25.20 | 25.61 |
Cs[Yb9Mn4Se18] | 39.14 | −54.22 | 17.71 | 18.05 |
Cs[Lu9Mn4Se18] | 16.06 | −6.28 | 11.53 | 11.84 |
To check the influence of the strong correlation effect at Mn sites, additionally GGA + U calculations were performed. The analyses indicated that the band gap of Cs[Tm9Mn4Se18] can be significantly widened by increasing U from 2 to 4 eV (see Fig. 8). Further increasing the effective Hubbard parameter does not alter significantly the calculated band gap value, which reaches a constant value of 1.68 eV. Consequently, the effective U (Ueff) of 5.0 eV was used for Mn in all the calculations carried out in this work. Similar Ueff was utilized in previous work.25
Fig. 8 The Hubbard U effect on the band gap of Cs[Tm9Mn4Se18], calculated by the GGA + U method. The Hubbard correction is made to the Mn 3d states. |
The on-site Coulomb correction was only applied to the Mn 3d electrons, which mostly increase the energy of empty 3d states, and widen the optical band gap. In order to simplify the calculation, the Tm 4f electrons were treated as core electrons. The total and partial DOSs of CsTm9Mn4Se18 were calculated, and the results revealed the semiconducting behavior and a band gap of about 1.67 eV (Fig. 9). The Ecal value is slightly smaller than Eexp (1.89 eV).
The valence band (VB) maximum near the Fermi level is mostly Se 4p states mixed with small contributions from Tm 5d and Mn 3d states, whereas the conduction band (CB) minimum is dominated by Tm 5d and Mn 3d states with minor contributions from Se 4p states. Cs atoms almost make no contribution around EF and act as electron donors to stabilize the structure. Accordingly, the optical absorptions are mainly assigned the charge transfer from Se 4p to Tm 5d and Mn 3d states.
Footnote |
† Electronic supplementary information (ESI) available: The cif file, experimental and theoretical methods, and additional tables and figures. CCDC 1044644–1044648. See DOI: 10.1039/c4qi00202d |
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