Physical properties of the new intermetallic compound Pr₂AgSi₃

Several intermetallics in the R₂TX₃ series (R = rare earth ion, T = transition metal and X = Si, Ge, In, etc.) have attracted particular attention due to their fascinating structural, magnetic and physical properties. They are mainly reported to crystallize in the ThSi₂ structure types (α and β phases). In these R₂TX₃ compounds, the magnetic moment is usually only carried by the R ions and is typically of a local-moment nature. This is due to the fact that the T and X ions occupy randomly the 2d wyckoff position. Moreover, these R atoms have a triangular arrangement in the crystal structure. This favors the occurence of geometric frustration in this class of materials^1–3 and the occurence of competition between antiferromagnetic and ferromagnetic interactions. Geometrical frustration arises in the presence of antiferromagnetic interactions and provides a bridge to obtain and understand the mechanisms behind competing/coexisting orders. In compounds such as Ce₂CuSi₃² and Ce₂NiGe₃,⁴ frustration and disorder have been proven to play a crucial role in the formation of a spin-glass state. Pr₂CuSi₃ and Nd₂CuSi₃⁵ were also classified as spin-glass materials with the formation of ferromagnetic clusters. In U₂IrSi₃,⁶ a topological disorder and frustration in the geometry of the lattice have led to the formation of magnetic clusters. The compound Ce₂PdGe₃⁷ revealed a complicated magnetic behavior and has been found to be of potential interest for the search of a quantum phase transition. Tb₂PdSi₃ has been called “exotic magnetic material” by Paulose et al.⁸ due to the huge changes in its magnetic properties with the variation of external parameters such as external field and temperature. Despite all the studies carried out on R₂TX₃ series with T = Pd, Cu and Ni for instance, relatively less work have been done on Ag based compounds. Among others, we have a systematic study of the R₂AgIn₃ series by Siouris et al.^9,10 and a structural and physical properties report on R₂AgGe₃ with (R= Ce, Pr, Nd) by Sarkar et al.¹¹ Moreover, the efficient hybridization between the localized 4f orbitals and the 4d and 5d elements (among which Ag) usually give rise to unusual and anomalous properties. These properties found in 2-1-3 materials make them interesting and provide our motivation to investigate the properties of the new intermetallic compound Pr₂AgSi₃.

A polycrystalline sample of Pr₂AgSi₃ was synthesized by arc-melting stoichiometric amounts of Pr, Ag, and Si (2:1:3 ratio) (99.99 weight-% purity) under high purity Argon gas. The obtained ingot was turned and remelted several times to ensure homogeneity. The final ingot was then wrapped in tantalum foil, sealed under vacuum in a clean quartz tube and annealed at 1100 ^rc C for two weeks.

Based upon a powder X-ray diffraction survey on a pulverized portion of the synthesized sample, the compound was found to form in the tetragonal α-ThSi₂ structure with space group I4₁/amd. Magnetic and physical properties such as magnetic susceptibility, specific heat and electrical resistivity were measured in the temperature range of 1.8 K - 300 K and applied fields up to 5T using a Physical Property Measurement System (PPMS) and Dynacool instrument, both from Quantum Design (San Diego). The temperature dependence magnetization is measured following the zero field cooled (ZFC) and field cooled (FC) protocols described in Ref. 12.

The Rietveld structural refinement of the room-temperature powder XRD pattern obtained using FULLPROF software is displayed in Fig. 1.

The sample forms in the tetragonal α-ThSi₂ structure (space group I4₁/amd) with lattice parameters a = 4.193(2) Å and c = 14.373(2) Å. In this structure with two Wyckoff positions (4a for Pr and 8e for Ag and Si), the Pr atoms occupy the Th site while the Ag and Si atoms of the compound are mixed on the Si site. The atomic site occupations and positions in the unit cell as found from the diffraction profile refinement are listed in Table I.

Figure 2 shows the temperature dependence of dc magnetic susceptibility (χ(T)) and inverse magnetic susceptibility measured under field cooled protocol in an applied magnetic field of H = 1 T for Pr₂AgSi₃. The linear paramagnetic region above 47 K of the inverse magnetic susceptibility has been fitted using the Curie-Weiss law: χ = N_Aμ_eff2/3k_B(T − θ_P), where N_A is the Avogadro number and k_B the Boltzmann constant. The least-squares fit yields an effective magnetic moment μ_eff = 3.56 μ_B which is very close to the theoretical value of 3.58 μ_B for the free ion Pr3 + . Therefore, the 4f- ions are the only species contributing to the magnetic properties of the system. The positive paramagnetic Curie-Weiss temperature θ_P = 8.49 K suggests the predominance of ferromagnetic interactions in the compound. As the system is cooled below 47 K, the system undergoes a paramagnetic to ferromagnetic transition with T_c ≈ 13 K as shown in Fig. 2. The inset (b) shows the hysteresis M-H curve obtained at 2 K which is below the transition temperature T_c. The compound exhibits a hysteresis loop at 2 K with a well defined magnetic remanence and coercivity. It shows a maximum magnetic moment of 1.2 μ_B/Pr at 9 T and a coercive field of 0.09 T. This hysteresis loop due to ferromagnetic ordering confirms the observation made in the susceptibility data. To further investigate the observed ferromagnetic transition, the temperature variation of magnetization was measured under different applied magnetic fields and is presented in Fig. 3. As seen in the figure, the ZFC and FC magnetization show an irreversibility behavior which depends on the applied magnetic field. Similar behavior has been observed in other R₂TX₃ compounds.^5,6

The temperature-dependent specific heat of Pr₂AgSi₃ and its isostructural nonmagnetic reference La₂AgSi₃ are shown in the main panel of Fig. 4. The zero field specific heat of Pr₂AgSi₃ shows a lambda peak shape around 13 K which is consistent with the transition observed in the magnetic susceptibility data. The polycrystalline sample of La₂AgSi₃ was also synthesized and found to form isostructural to the title compound Pr₂AgSi₃. We further notice that no peak is observed in La₂AgSi₃, indicating the absence of magnetic ordering in this system. The 4f-magnetic contribution (C_4f) to the specific heat of the compound was deduced by subtracting the specific heat of La₂AgSi₃ from that of Pr₂AgSi₃ and is presented in inset (a) of Fig. 4. A necessary mass correction associated with the molar mass difference of these two compounds was performed prior to the subtraction using the method adopted by Bouvier et al.¹³ The obtained C_4f data also shows a lambda type peak around the ordering temperature.

The magnetic entropy (S_4f) associated with the 4f-electrons is calculated from the C_P/T vs. T plot. It was derived by integrating C_P/T with respect to T and is shown in the inset (b) of Fig. 4. As observed, S_4f gradually increases with temperature and has a tendency to saturate at T > 100 K. The S_4f released at T_C ≈ 13 K is approximately R ln 2 = 5.76 J/(Pr.mole.K). This revealed that Pr3 + has a doublet magnetic ground state in this compound. Towards room temperature, S_4f of Pr₂AgSi₃ saturates at R ln(9) for J = 4 of Pr3 + . This result indicates that the full Hund’s rule crystal-field split multiplet of trivalent Pr is occupied at ≈ 300 K.

The temperature dependent electrical resistivity ρ(T) measured in zero applied field for Pr₂AgSi₃ is presented in the main panel of Fig. 5. A metallic behavior is observed which is characterized by a gradual decrease of resistivity with decreasing of temperature down to about 15 K. The resistivity reaches a constant value of ρ₀ ≈ 218 μΩ.cm in the normal state which is known as the residual resistivity from which we can estimate the residual resistivity ratio RRR ≡ ρ(300K)/ρ(2K) that gives an idea about the sample quality. This gives a relatively low value RRR = 1.14 which suggests that the compound is not of highly crystalline quality and thus indicates the presence of some low metallurgical defects. Similar values have been found in several metallic compounds.

At low temperatures, an anomaly is also seen in the form of a knee in ρ(T) at the transition temperature of 13 K. Thereafter, ρ decreases with decreasing temperature, which confirms the ferromagnetic nature of the transition. This is in good agreement with the χ(T) and C_P(T) data. The inset of Fig. 5 shows the low temperature data of ρ vs. T in the range of 2 K to 30 K.

We report on the magnetic and physical properties of the new stoichiometric compound Pr₂AgSi₃. We confirmed based on χ(T), C_P(T) and ρ(T) the presence of long-range ferromagnetic ordering below T_c ≈ 13 K. Further magnetic studies are needed to describe in detail the phase transition observed in Pr₂AgSi₃.

DFR thanks OWSD and SIDA for the fellowship towards Ph.D. studies. BS thanks UJGES for the postdoctoral fellowship. AMS thanks the URC/FRC of UJ and the SA NRF (93549) for financial support.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.