Nanolaminated materials including magnetic elements are of special interest for commonly observed nontrivial magnetic characteristics and as potential precursors for 2D materials. Here, we explore ...the previously unknown layered phase M2Al2C3, where M = Sc and Er. Sc2Al2C3 was synthesized as single crystals of ∼mm2 size, and its structure was determined by single crystal X-ray diffraction and scanning transmission electron microscopy. Evaluation of phase stability and possible vacancy formation based on first-principles calculations confirms the attained phase and suggests full occupancy on both the Al and C sites. Potential realization of the hypothetical phase Y2Al2C3 is also proposed. Furthermore, we also demonstrate that Er2Al2C3 can be synthesized in powder form, providing experimental evidence for stoichiometries based on rare earth elements, which, in turn, suggests possible incorporation of other lanthanides.
We report the discovery of a new hexagonal Mo2Ga2C phase, wherein two Ga layers – instead of one – are stacked in a simple hexagonal arrangement in between Mo2C layers. It is reasonable to assume ...this compound is the first of a larger family.
In 2017, we discovered quaternary i-MAX phasesatomically layered solids, where M is an early transition metal, A is an A group element, and X is Cwith a (M1 2/3M2 1/3)2AC chemistry, where the M1 ...and M2 atoms are in-plane ordered. Herein, we report the discovery of a class of magnetic i-MAX phases in which bilayers of a quasi-2D magnetic frustrated triangular lattice overlay a Mo honeycomb arrangement and an Al Kagomé lattice. The chemistry of this family is (Mo2/3RE1/3)2AlC, and the rare-earth, RE, elements are Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. The magnetic properties were characterized and found to display a plethora of ground states, resulting from an interplay of competing magnetic interactions in the presence of magnetocrystalline anisotropy.
Boridene: a 2D boride
A range of two-dimensional (2D) materials, including graphene and hexagonal boron nitride, have been synthesized and studied because of the unusual properties that occur when ...one dimension becomes very small. MXenes are a family of materials made of layers of inorganic transition metal carbides and nitrides that are a few atoms thick and are manufactured by selective etching. Attempts to make similar boridene materials have been challenging because of the reactive nature of boride phases and because the parent materials tend to dissolve rather than selectively etch. Zhou
et al
. synthesized boridene in the form of single-layer 2D molybdenum boride sheets by selective etching in aqueous hydrofluoric acid to produce sheets with ordered metal vacancies, opening up an additional family of materials for study. —MSL
The two-dimensional transition metal boride is synthesized by removal of select layers in laminated borides.
Extensive research has been invested in two-dimensional (2D) materials, typically synthesized by exfoliation of van der Waals solids. One exception is MXenes, derived from the etching of constituent layers in transition metal carbides and nitrides. We report the experimental realization of boridene in the form of single-layer 2D molybdenum boride sheets with ordered metal vacancies, Mo
4/3
B
2-x
T
z
(where T
z
is fluorine, oxygen, or hydroxide surface terminations), produced by selective etching of aluminum and yttrium or scandium atoms from 3D in-plane chemically ordered (Mo
2/3
Y
1/3
)
2
AlB
2
and (Mo
2/3
Sc
1/3
)
2
AlB
2
in aqueous hydrofluoric acid. The discovery of a 2D transition metal boride suggests a wealth of future 2D materials that can be obtained through the chemical exfoliation of laminated compounds.
Nanolaminated materials including magnetic elements are of special interest for commonly observed nontrivial magnetic characteristics and as potential precursors for 2D materials. Here, we explore ...the previously unknown layered phase M 2 Al 2 C 3 , where M = Sc and Er. Sc 2 Al 2 C 3 was synthesized as single crystals of ∼mm 2 size, and its structure was determined by single crystal X-ray diffraction and scanning transmission electron microscopy. Evaluation of phase stability and possible vacancy formation based on first-principles calculations confirms the attained phase and suggests full occupancy on both the Al and C sites. Potential realization of the hypothetical phase Y 2 Al 2 C 3 is also proposed. Furthermore, we also demonstrate that Er 2 Al 2 C 3 can be synthesized in powder form, providing experimental evidence for stoichiometries based on rare earth elements, which, in turn, suggests possible incorporation of other lanthanides.
We report the results of magnetization, heat capacity, and neutron diffraction measurements on (Mo2/3RE1/3)2AlC with RE=Dy and Tb. Temperature and field-dependent magnetization as well as heat ...capacity were measured on a powder sample and on a single crystal allowing the construction of the magnetic field-temperature phase diagram. To study the magnetic structure of each magnetic phase, we applied neutron diffraction in a magnetic field up to 6 T. For (Mo2/3Dy1/3)2AlC in zero field, a spin density wave is stabilized at 16 K, with antiferromagnetic ordering at 13 K. Furthermore, we identify the coexistence of ferromagnetic and antiferromagnetic phases induced by magnetic fields for both RE=Tb and Dy. The origin of the field induced phases is resulting from the competing ferromagnetic and antiferromagnetic interactions.
Abstract
We report the results of magnetization, heat capacity, and neutron diffraction measurements on (Mo
2/3
RE
1/3
)
2
AlC with RE = Dy and Tb. Temperature and field-dependent magnetization as ...well as heat capacity were measured on a powder sample and on a single crystal allowing the construction of the magnetic field-temperature phase diagram. To study the magnetic structure of each magnetic phase, we applied neutron diffraction in a magnetic field up to 6 T. For (Mo
2/3
Dy
1/3
)
2
AlC in zero field, a spin density wave is stabilized at 16 K, with antiferromagnetic ordering at 13 K. Furthermore, we identify the coexistence of ferromagnetic and antiferromagnetic phases induced by magnetic fields for both RE = Tb and Dy. The origin of the field induced phases is resulting from the competing ferromagnetic and antiferromagnetic interactions.
This thesis is focused towards the synthesis and characterization of novel nanolaminated materials in primarily bulk (powder) form. Of particular interest is magnetic materials, or laminates that can ...be used as precursor for two-dimensional (2D) materials. 2D materials typically display a large surface-to-volume ratio, and as such they are very promising for applications within energy storage and catalysis. A more recently discovered family of 2D transition metal carbides/nitrides, called MXenes, are currently attracting a lot of attention. MXenes are produced by selective etching of parent 3D nanolaminates, so called MAX phases, facilitating removal of selected atomic layers, and formation of 2D sheets. In my work on new nanolaminates as precursors for 2D materials, I have synthesized (Mo2/3Sc1/3)2AlC and have studied its crystal structure. It was found that Mo and Sc are chemically ordered in the metal layers, with the in-plane ordering motivating the notation i-MAX for this new type of MAX phase alloy. By selective etching of Sc and Al, we thereafter produced a 2D materials with ordered vacancies, Mo1.33C, and studied the electrochemical properties. It was found that the material displayed a high capacitance, ~1200 F cm-3, which is 65% higher that the counterpart without vacancies, Mo2C. I also synthesized a previously not known out-of-plane ordered Mo2ScAlC2 MAX phase. By selective etching of Al, we produced a 2D material, Mo2ScC2, which is correspondingly ordered in the out-of-plane direction. Another related laminated material was also discovered and synthesized, Sc2Al2C3, and its crystal structure was determined. The material is potentially useful for conversion into a 2D material. I have also shown that Sc2Al2C3 is an example of a series of materials with the same crystal structure, with Sc replaced by other metals. Magnetic materials are used in many applications, such as for data storage devices. In particular, layered magnetic materials are of interest due to their anisotropic structure and potential formation of interesting magnetic characteristics. I have been synthesizing and characterizing magnetic nanolaminates, starting with the (V,Mn)3GaC2 MAX phase in the form of an epitaxial thin film. Analysis of the magnetic behavior showed a ferromagnetic response above room temperature I thereafter showed that our previously discovered family of i-MAX phases could be expanded with a subclass of ordered nanolaminates based on rare earth (RE) elements, of the general formula (Mo2/3RE1/3)2AlC , where RE=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. I studied their crystal structure by scanning transmission microscopy (STEM), X-ray diffraction (XRD), and neutron diffraction. We found that these phases can crystalize in three different structures, of space group C2/m, C2/c, and Cmcm, respectively. The magnetic behavior was studied and the magnetic structure of two materials could be determined. We suggest that the complex behavior identified is due to competing magnetic interaction and frustration. I also synthesized another rare earth-based nanolaminate, Mo4Ce4Al7C3. The crystal structure was investigated by single crystal X-ray diffraction and STEM. Magnetization analysis reveal a ferromagnetic ground state below 10.5 K. X-ray absorption near-edge structure provide evidence that Ce is in a mixed-valence state. X-ray magnetic circular dichroism shows that only one of the two Ce sites are magnetic.
We report the results of magnetization, heat capacity, and neutron diffraction measurements on (Mo2/3RE1/3)(2)AlC with RE = Dy and Tb. Temperature and field-dependent magnetization as well as heat ...capacity were measured on a powder sample and on a single crystal allowing the construction of the magnetic field-temperature phase diagram. To study the magnetic structure of each magnetic phase, we applied neutron diffraction in a magnetic field up to 6 T. For (Mo2/3Dy1/3)(2)AlC in zero field, a spin density wave is stabilized at 16 K, with antiferromagnetic ordering at 13 K. Furthermore, we identify the coexistence of ferromagnetic and antiferromagnetic phases induced by magnetic fields for both RE = Tb and Dy. The origin of the field induced phases is resulting from the competing ferromagnetic and antiferromagnetic interactions.
The recent discovery of chemical ordering in quaternary borides offers new ways of exploring properties and functionalities of these laminated phases. Here, we have synthesized and investigated ...chemical ordering of the laminated Mo4MnSiB2 (T2) phase, thereby introducing a magnetic element into the family of materials coined o-MAB phases. By X-ray diffraction and scanning transmission electron microscopy, we provide evidence for out-of-plane chemical ordering of Mo and Mn, with Mo occupying the 16l site and Mn preferentially residing in the 4c site. Mn and B constitute quasi-two-dimensional layers in the laminated material. We have therefore also studied the magnetic properties by magnetometry, and no sign of long-range magnetic order is observed. An initial assessment of the magnetic ordering has been further studied by density functional theory (DFT) calculations, and while we find an antiferromagnetic configuration to be the most stable one, ferromagnetic ordering is very close in energy.
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