In this study, we report on the valence state of Eu cations and the magnetic properties of 14-1-11 Zintl arsenides, Eu₁₄MAs₁₁ (M = Mg, Zn, Cd). ¹⁵¹Eu Mössbauer spectroscopic analysis indicates the presence of intrinsic Eu²⁺ and Eu³⁺ cations in Eu₁₄MgAs₁₁^^ and Eu₁₄CdAs₁₁. In contrast, Eu₁₄ZnAs₁₁ shows the presence of Eu³⁺ cations that can be identified as an impurity. Mixed valence in Eu₁₄MgAs₁₁ and Eu₁₄CdAs₁₁ significantly affects their transport properties, resulting in lower conductivity than the Zn compound. The zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibility of Eu₁₄MAs₁₁ (M = Mg, Zn, Cd) exhibits paramagnetic behavior at high temperatures. While Eu₁₄MgAs₁₁ exhibits antiferromagnetic ordering at low temperatures, the ZFC and FC magnetic susceptibilities of Eu₁₄ZnAs₁₁ and Eu₁₄CdAs₁₁ diverge at low temperatures, indicating complex magnetic ordering.

Synthesis and Spark Plasma Sintering

Eu₁₄MAs₁₁ (M = Mg, Zn, Cd) was synthesized using a binary precursor approach similar to that described by Islam et al.¹² Stoichiometric amounts of europium ingot (Stanford Advanced Materials, 99.99%) and arsenic chips (Johnson Matthey Chemicals, 99.9999%) were ball-milled in a 65 cm³ stainless steel grinding vial with two stainless steel balls of 12.7 mm using a SPEX 8000D Mixer/Mill for 30 minutes. The mixture was then scraped with a chisel and milled for an additional hour to create a homogeneous powder. The resulting powder was sealed in a tantalum tube and jacketed inside a quartz tube under vacuum. The sealed quartz tube was heated to 850 °C at a rate of 100 °C/hour and annealed at this temperature for 12 hours. Other binary precursors, namely Mg₃As₂, Zn₃As₂, and Cd₃As₂, were synthesized in a similar manner by weighing out stoichiometric amounts of Mg turnings (Strem Chemicals, 99.8%), Zn shots (Alfa Aesar, 99.999%), and Cd shots (Luciteria Science, 99.999%), respectively. These mixtures were heated at a rate of 100 ˚C/h to 650 °C and then annealed for 12 hours.

Ternary phases Eu₁₄ZnAs₁₁ and Eu₁₄CdAs₁₁ were synthesized by weighing out stoichiometric amounts of EuAs, Eu metal, and Zn₃As₂ or Cd₃As₂, respectively. For the synthesis of Eu₁₄MgAs₁₁, Mg was provided using Mg₃As₂, while EuH₂ (American Elements, 99.9%) was used in place of Eu metal in the required amount. The mixture was then ball-milled and scraped in a manner similar to the previous procedures. The homogeneous powder was annealed at 1100°C for 96 hours.

The annealed powder was pressed into dense pellets using a Dr. Sinter Junior Spark Plasma Sintering system (Fuji Electronic Industrial Co., Ltd.) in a 50% vacuum/Argon environment. For densification, the powder was loaded into a 12.7 mm graphite die and cold pressed to 6 kN. The applied force was gradually increased from 6 kN to 10.5 kN. During this increase, the powder was heated to 850 °C over 10 minutes. It was then heated to 900 °C within 1 minute and held at this temperature for 30 minutes to ensure sample consolidation. The resulting pellets achieved a density greater than 98%, as determined by Archimedes' principle.

Powder X-ray Diffraction

Powder X-ray diffraction data on powder samples were collected at room temperature using a Bruker D8 ADVANCE ECO instrument with Cu Kα radiation. The measurements were taken in the 2θ range of 5° to 80°, with a step size of 0.02°.

¹⁵¹Eu Mössbauer Spectroscopy

The sample for ¹⁵¹Eu Mössbauer spectroscopy was prepared by mixing 70 mg/cm² of sieved powder with boron nitride powder in a glove box. The mixture was then sandwiched between Kapton tape and loaded onto a 14 mm diameter holder. ¹⁵¹Eu Mössbauer spectroscopy measurements were conducted using a 40 mCi ¹⁵¹SmF₃ source with 21.5 keV gamma radiation, and a Tl@NaI detector. The sample was placed in a Janis SHI-850 closed-cycle cryocooler for the analysis. Isomer shifts are reported relative to the source.

Magnetism

The magnetic properties of pieces of the pressed pellets were analyzed using a Quantum Design magnetic property measurement system (MPMS) under zero-field-cooled (ZFC) and field-cooled (FC) conditions, ranging from 2 K to 300 K and up to 7 T. Data at 1000 Oe were collected in the FC mode.